Do not consider PortUnreachableException to require channel closure

Motivation:

When a datagram packet is sent to a destination where nobody actually listens to,
the server O/S will respond with an ICMP Port Unreachable packet.
The ICMP Port Unreachable packet is translated into PortUnreachableException by JDK.
PortUnreachableException is not a harmful exception that prevents a user from sending a datagram.
Therefore, we should not close a datagram channel when PortUnreachableException is caught.

Modifications:

- Do not close a channel when the caught exception is PortUnreachableException.

Result:

A datagram channel is not closed unexpectedly anymore.
Add EDNS support to DnsQueryEncoder

Motivation:

DnsQueryEncoder does not encode the 'additional resources' section at all, which contains the pseudo-RR as defined in RFC 2671.

Modifications:

- Modify DnsQueryEncoder to encode the additional resources
- Fix a bug in DnsQueryEncoder where an empty name is encoded incorrectly

Result:

A user can send an EDNS query.
Name resolver API and DNS-based name resolver

Motivation:

So far, we relied on the domain name resolution mechanism provided by
JDK.  It served its purpose very well, but had the following
shortcomings:

- Domain name resolution is performed in a blocking manner.
  This becomes a problem when a user has to connect to thousands of
  different hosts. e.g. web crawlers
- It is impossible to employ an alternative cache/retry policy.
  e.g. lower/upper bound in TTL, round-robin
- It is impossible to employ an alternative name resolution mechanism.
  e.g. Zookeeper-based name resolver

Modification:

- Add the resolver API in the new module: netty-resolver
- Implement the DNS-based resolver: netty-resolver-dns
  .. which uses netty-codec-dns
- Make ChannelFactory reusable because it's now used by
  io.netty.bootstrap, io.netty.resolver.dns, and potentially by other
  modules in the future
  - Move ChannelFactory from io.netty.bootstrap to io.netty.channel
  - Deprecate the old ChannelFactory
  - Add ReflectiveChannelFactory

Result:

It is trivial to resolve a large number of domain names asynchronously.
Fix test failures in ProxyHandlerTest

Motivation:

The default name resolver attempts to resolve the bad host name (destination.com) and actually succeeds, making the ProxyHandlerTest fail.

Modification:

Use NoopNameResolverGroup instead.

Result:

ProxyHandlerTest passes again.
Fix resource leak in DnsNameResolver
Fix the deprecation warning in DnsNameResolver
Upgrading HTTP/2 hpack to latest version

Motivation:

Twitter hpack has upgraded to 0.9.1, we should upgrade to the latest.

Modifications:

Updated the parent pom to specify the dependency version. Updated the
http2 pom to use the version specified by the parent.

Result:

HTTP/2 updated to the latest hpack release.
Fix another resource leak in DnsNameResolver

- Fix a bug in cache expiration task; wrong object was being released
- Added more sanity checks when caching an entry
Make TestUtils.getFreePort() check both TCP and UDP

Motivation:

We see occational failures in the datagram tests saying 'address already
in use' when we attempt to bind on a port returned by
TestUtils.getFreePort().

It turns out that TestUtils.getFreePort() only checks if TCP port is
available.

Modifications:

Also check if UDP port is available, so that the datagram tests do not
fail because of the 'address already in use' error during a bind
attempt.

Result:

Less chance of datagram test failures
Fix an infinite loop when writing a zero-length FileRegion

Related: #2964

Motivation:

Writing a zero-length FileRegion to an NIO channel will lead to an
infinite loop.

Modification:

- Do not write a zero-length FileRegion by protecting with proper 'if'.
- Update the testsuite

Result:

Another bug fixed
Make Bootstrap and ServerBootstrap fully overridable

Related: #2034

Motivation:

Some users want to mock Bootstrap (or ServerBootstrap), and thus they
should not be final but be fully overridable and extensible.

Modifications:

Remove finals wherever possible

Result:

@daschl is happy.
Master - Fix "=" character in HttpPostRequestDecoder

Motivation
Issue #3004 shows that "=" character was not supported as it should in
the HttpPostRequestDecoder in form-data boundary.

Modifications:
Add 2 methods in StringUtil

split with maxPart: String split with a max parts limit only (to prevent
multiple '=' to be source of extra split while not needed when using limit of 2)
substringAfter: String part after delimiter (since first part is not needed)
Use those methods in HttpPostRequestDecoder. Change and the
HttpPostRequestDecoderTest to check using a boundary beginning with "=".

Results:
The fix implies more stability and fix the issue.
Master - Fix "=" character in HttpPostRequestDecoder