Correctly release buffer when testing DelegatingHttp2ConnectionHandler
Better implementation of AttributeMap and also add hasAttr(...). See [#2439]

Motivation:
The old DefaultAttributeMap impl did more synchronization then needed and also did not expose a efficient way to check if an attribute exists with a specific key.

Modifications:
* Rewrite DefaultAttributeMap to not use IdentityHashMap and synchronization on the map directly. The new impl uses a combination of AtomicReferenceArray and synchronization per chain (linked-list). Also access the first Attribute per bucket can be done without any synchronization at all and just uses atomic operations. This should fit for most use-cases pretty weel.
* Add hasAttr(...) implementation

Result:
It's now possible to check for the existence of a attribute without create one. Synchronization is per linked-list and the first entry can even be added via atomic operation.
Preparation for porting OpenSSL support in 3.10

- Add licenses and dependencies
Add an OpenSslEngine and the universal API for enabling SSL

Motivation:

Some users already use an SSLEngine implementation in finagle-native. It
wraps OpenSSL to get higher SSL performance.  However, to take advantage
of it, finagle-native must be compiled manually, and it means we cannot
pull it in as a dependency and thus we cannot test our SslHandler
against the OpenSSL-based SSLEngine.  For an instance, we had #2216.

Because the construction procedures of JDK SSLEngine and OpenSslEngine
are very different from each other, we also need to provide a universal
way to enable SSL in a Netty application.

Modifications:

- Pull netty-tcnative in as an optional dependency.
  http://netty.io/wiki/forked-tomcat-native.html
- Backport NativeLibraryLoader from 4.0
- Move OpenSSL-based SSLEngine implementation into our code base.
  - Copied from finagle-native; originally written by @jpinner et al.
  - Overall cleanup by @trustin.
- Run all SslHandler tests with both default SSLEngine and OpenSslEngine
- Add a unified API for creating an SSL context
  - SslContext allows you to create a new SSLEngine or a new SslHandler
    with your PKCS#8 key and X.509 certificate chain.
  - Add JdkSslContext and its subclasses
  - Add OpenSslServerContext
- Add ApplicationProtocolSelector to ensure the future support for NPN
  (NextProtoNego) and ALPN (Application Layer Protocol Negotiation) on
  the client-side.
- Add SimpleTrustManagerFactory to help a user write a
  TrustManagerFactory easily, which should be useful for those who need
  to write an alternative verification mechanism. For example, we can
  use it to implement an unsafe TrustManagerFactory that accepts
  self-signed certificates for testing purposes.
- Add InsecureTrustManagerFactory and FingerprintTrustManager for quick
  and dirty testing
- Add SelfSignedCertificate class which generates a self-signed X.509
  certificate very easily.
- Update all our examples to use SslContext.newClient/ServerContext()
- SslHandler now logs the chosen cipher suite when handshake is
  finished.

Result:

- Cleaner unified API for configuring an SSL client and an SSL server
  regardless of its internal implementation.
- When native libraries are available, OpenSSL-based SSLEngine
  implementation is selected automatically to take advantage of its
  performance benefit.
- Examples take advantage of this modification and thus are cleaner.
Optimize SslHandler in an OpenSslEngine-friendly way

Motivation:

Previous fix for the OpenSslEngine compatibility issue (#2216 and
18b0e95659c057b126653bad2f018a8ce5385255) was to feed SSL records one by
one to OpenSslEngine.unwrap().  It is not optimal because it will result
in more JNI calls.

Modifications:

- Do not feed SSL records one by one.
- Feed as many records as possible up to MAX_ENCRYPTED_PACKET_LENGTH
- Deduplicate MAX_ENCRYPTED_PACKET_LENGTH definitions

Result:

- No allocation of intemediary arrays
- Reduced number of calls to SSLEngine and thus its underlying JNI calls
- A tad bit increase in throughput, probably reverting the tiny drop
  caused by 18b0e95659c057b126653bad2f018a8ce5385255
Fix JDK 8 compatibility issue with OpenJdkSelfSignedCertGenerator

- X509CertInfo.setSubject/setIssuer() requires X500Name instead of
  CertificateSubjectName/CertificateIssuerName.
Fix buffer leaks during PEM to KeyStore conversion
Add netty-tcnative dependency to netty-example

Motivation:

It's useful to have netty-tcnative dependency in netty-example because
we can play with OpenSslEngine from our IDE.

Modifications:

Add netty-tcnative to example/pom.xml
Reduce memory usage of SslHandler when OpenSslEngine is in use

Motivation:

JDK's SSLEngine.wrap() requires the output buffer to be always as large as MAX_ENCRYPTED_PACKET_LENGTH even if the input buffer contains small number of bytes.  Our OpenSslEngine implementation does not have such wasteful behaviot.

Modifications:

If the current SSLEngine is OpenSslEngine, allocate as much as only needed.

Result:

Less peak memory usage.
Upgrade os-maven-plugin to fix the build issue
Add JVM crash logs to .gitignore
Update os-maven-plugin again to address an IDEA integration issue
Work around the JVM crash that occurs when cipher suite uses GCM

Motivation:

For an unknown reason, JVM of JDK8 crashes intermittently when
SslHandler feeds a direct buffer to SSLEngine.unwrap() *and* the current
cipher suite has GCM (Galois/Counter Mode) enabled.

Modifications:

Convert the inbound network buffer to a heap buffer when the current
cipher suite is using GCM.

Result:

JVM does not crash anymore.
Limit the number of bytes to copy per Unsafe.copyMemory()

Motivation:

During a large memory copy, safepoint polling is diabled, hindering
accurate profiling.

Modifications:

Only copy up to 1 MiB per Unsafe.copyMemory()

Result:

Potentially more reliable performance
Checkstyle
Clean up example execution

Motivation:

- example/pom.xml has quite a bit of duplication.
- We expect that we depend on npn-boot in more than one module in the
  near future. (e.g. handler, codec-http, and codec-http2)

Modification:

- Deduplicate the profiles in example/pom.xml
- Move the build configuration related with npn-boot to the parent pom.
- Add run-example.sh that helps a user launch an example easily

Result:

- Cleaner build files
- Easier to add a new example
- Easier to launch an example
- Easier to run the tests that relies on npn-boot in the future
Fix a problem where all classes are compiled again

Motivation:

Due to a known problem[1] of maven-compiler-plugin, our build always
compiles everything from scratch, which is waste of time.

Modifications:

Exclude package-info.java from the source list.

Result:

Much shorter build time.

[1]: https://jira.codehaus.org/browse/MCOMPILER-205
Fix a build problem with JDK 8

Motivation:

Build fails with JDK 8 because npn-boot does not work with JDK 8

Modifications:

Do not specify bootclasspath when on JDK 8

Result:

Build is green again.
Clean up the execution mechanism of examples

Motivation:

- There's no way to pass an argument to an example.
- Assigning a Maven profile for each example is an overkill.
  It makes the pom.xml crowded.

Modifications:

- Remove example profiles from example/pom.xml
- Keep the list of examples in run-example.sh
- run-example.sh passes all options to exec-maven-plugin.
  For example, we can now do this:

    ./run-example.sh -Dssl -Dport=443 http-server

Result:

- It's much easier to add a new example and provide an easy way to
  launch it.
- We can still pass an arbitrary argument to the example being launched.
  (I'll update all examples to make them get their options from system
  properties rather than from args[].
Improve run-example.sh

- More usage example
- Newlines for prettier output