[#2353] Use a privileged block to get ClassLoader and System property if needed

Motivation:
When using System.getProperty(...) and various methods to get a ClassLoader it will fail when a SecurityManager is in place.

Modifications:
Use a priveled block if needed. This work is based in the PR #2353 done by @anilsaldhana .

Result:
Code works also when SecurityManager is present
[#2370] Periodically check for not alive Threads and free up their ThreadPoolCache

Motivation:
At the moment we create new ThreadPoolCache whenever a Thread tries either allocate or release something on the PooledByteBufAllocator. When something is released we put it then in its ThreadPoolCache. The problem is we never check if a Thread is not alive anymore and so we may end up with memory that is never freed again if a user create many short living Threads that use the PooledByteBufAllocator.

Modifications:
Periodically check if the Thread is still alive that has a ThreadPoolCache assinged and if not free it.

Result:
Memory is freed up correctly even for short living Threads.
[#2371] Fix Potential data corruption in EpollSocketChannel when writing to the Channel

Motivation:
We sometimes see data corruption when writing to the EpollSocketChannel.

Modifications:
Correctly update the position of the ByteBuffer after something was written.

Result:
Fix data-corruption which could happen on partial writes
Adding support for HTTP/2 initialization handshake.

Motivation:

The current HTTP/2 support does not properly comply with the HTTP/2 spec
wrt startup.

Modifications:

Changed the frame codec as well as the connection handler to support
exchange of the connection preface, followed immediately by an initial
settings frame.

Result:

The HTTP/2 initialization handshake will be in compliance with the spec.
Will need more work to support the upgrade protocols, however :)
Add a Example for Memcache Binary Codec

Motivation:
Currently, there exists no example which shows how to use the memcache binary
protocol.

Modifications:
Add an example client and client handler to show how to utilize the binary
protocol in a memcache client with a simple interactive shell.

Result:
Users looking for an example can now start off with the provided one.
[#2390] Minimize memory usage of NioDatagramChannel

Motivation:
At the moment we create a HashMap that holds the MembershipKeys for multicast with every NioDatagramChannel even when most people not need it at al

Modifications:
Lazy create the HashMap when needed.

Result:
Less memory usage and less object creation
Fix error that causes (up to) double memory usage

Motivation:

PoolArena's 'normalizeCapacity' function was micro-optimized some
time ago to remove a while loop. However, there was a change of
behavior in the function as a result. Capacities passed into it
that are already powers of 2 (and >= 512) are doubled in size. So
if I ask for a buffer with a capacity of 1024, I will get back one
that actually uses 2048 bytes (stored in maxLength).

Aligning to powers of two for book keeping ease is reasonable,
and if someone tries to expand a buffer, you might as well use some
of the previously wasted space. However, since this distinction
between 'easily expanded' and 'costly to expand' space is not
supported at all by the APIs, I cannot imagine this change to
doubling is desirable or intentional.

This is especially costly when using composite buffers. They
frequently allocate components with a capacity that is a power of
2, and they never attempt to expand components themselves. The end
result is that heavy use of pool-backed composite buffers wastes
almost half of the memory pool (the smaller / initial components are
<512 and so are not affected by the off-by-one bug).

Modifications:

Although I find it difficult to believe that such an optimization
is really helpful, I left it in and fixed the off-by-one issue by
decrementing the value at the start.

I also added a simple test to both attempt to verify that the
decrement fixes the issue without introducing any other change, and
to make it easy for a reviewer to test the existing behavior. PoolArena
does not seem to have much testing or testability support though so
the test is kind of a hack and will break for unrelated changes. I
suggest either removing it or factoring out the single non-static
portion of normalizeCapacity so that the fragile dummy PoolArena is
not required.

Result:

Pooled allocators will allocate less resources to the highly
inefficient and undocumented buffer section between length and
maxLength.

Composite buffers of non-trivial size that are backed by pooled
allocators will use about half as much memory.