Add io.netty.noJdkZlibEncoder system property

Related issue: #2821

Motivation:

There's no way for a user to change the default ZlibEncoder
implementation.

It is already possible to change the default ZlibDecoder implementation.

Modification:

Add a new system property 'io.netty.noJdkZlibEncoder'.

Result:

A user can disable JDK ZlibEncoder, just like he or she can disable JDK
ZlibDecoder.
Check noJdkZlibEncoder before comparing other parameters

.. for consistency
[#2823] Writing DefaultFileRegion with EpollSocketChannel may cause hang

Motivation:

In EpollSocketchannel.doWriteFileRegion(...) we need to make sure we write until sendFile(...) returns either 0 or all is written. Otherwise we may not get notified once the Channel is writable again.

This is the case as we use EPOLL_ET.

Modifications:

Always write until either sendFile returns 0 or all is written.

Result:

No more hangs when writing DefaultFileRegion can happen.
Don't ignore maxCapacity if it's not a power of 2

Motivation:

This fixes bug #2848 which caused Recycler to become unbounded and cache infinite number of objects with maxCapacity that's not a power of two. This can result in general sluggishness of the application and OutOfMemoryError.

Modifications:

The test for maxCapacity has been moved out of test to check if the buffer has filled. The buffer is now also capped at maxCapacity and cannot grow over it as it jumps from one power of two to the other.

Additionally, a unit test was added to verify maxCapacity is honored even when it's not a power of two.

Result:

With these changes the user is able to use a custom maxCapacity number and not have it ignored. The unit test assures this bug will not repeat itself.
[#2843] Add test-case to show correct behavior of ByteBuf.refCnt() and ByteBuf.release(...)

Motivation:

We received a bug-report that the ByteBuf.refCnt() does sometimes not show the correct value when release() and refCnt() is called from different Threads.

Modifications:

Add test-case which shows that all is working like expected

Result:

Test-case added which shows everything is ok.
Allow to write CompositeByteBuf directly via EpollDatagramChannel. Related to [#2719]

Motivation:

On linux it is possible to use the sendMsg(...) system call to write multiple buffers with one system call when using datagram/udp.

Modifications:

- Implement the needed changes and make use of sendMsg(...) if possible for max performance
- Add tests that test sending datagram packets with all kind of different ByteBuf implementations.

Result:

Performance improvement when using CompoisteByteBuf and EpollDatagramChannel.
Add support for sendmmsg(...) and so allow to write multiple DatagramPackets with one syscall. Related to [#2719]

Motivation:

On linux with glibc >= 2.14 it is possible to send multiple DatagramPackets with one syscall. This can be a huge performance win and so we should support it in our native transport.

Modification:

- Add support for sendmmsg by reuse IovArray
- Factor out ThreadLocal support of IovArray to IovArrayThreadLocal for better separation as we use IovArray also without ThreadLocal in NativeDatagramPacketArray now
- Introduce NativeDatagramPacketArray which is used for sendmmsg(...)
- Implement sendmmsg(...) via jni
- Expand DatagramUnicastTest to test also sendmmsg(...)

Result:

Netty now automatically use sendmmsg(...) if it is supported and we have more then 1 DatagramPacket in the ChannelOutboundBuffer and flush() is called.