Reset interrupted flag in SingleThreadEventLoopTest. Fixes #2841

Motivation:

Once the interrupted flag in SingleThreadEventLoopB was set it was never reset.
That way the eventloop thread was always in an interrupted state and all calls
to Thread.sleep/LockSupport.parkNanos would fail.

Modifications:

Reset interrupted flag.

Result:

Number of GC runs when executing the test is down to zero (from 20 - 50).
Looking at the heap dump shows >10x decrease in memory consumption e.g.
the number of ForkJoinTasks is down to 25k from 350k previously. Similar
is true for BlockingQueue.Node objects.
Fixing queuing bug in outbound flow control.

Motivation:

Outbound flow control does not properly remove the head of queue after
it's written. This will cause streams with multiple frames to get stuck
and not send all of the data.

Modifications:

Modified the DefaultHttp2OutboundFlowController to properly remove the
head of the pending write queue once a queued frame has been written.

Added an integration test that sends a large message to verify that all
DATA frames are properly collected at the other end.

Result:

Outbound flow control properly handles several queued messages.
Handler Unit Test Bug

Motivation:
There is a bug in the JettySslEngineTest where the interface receiving a message does not do a latch.countDown().
This causes each test to be subject to the CountDownLatch timeout period instead of being notified right when an event occurs.

Modifications:
- The JettySslEngineTest message handler will call the appropriate latch.countDown after a message is received

Result:
JettySslEngineTest will not be subject to waiting the timeout period even if the message is correctly received
[#2867] Workaround performance issue with IPv4-mapped-on-IPv6 addresses

Motivation:

InetAddress.getByName(...) uses exceptions for control flow when try to parse IPv4-mapped-on-IPv6 addresses. This is quite expensive.

Modifications:

Detect IPv4-mapped-on-IPv6 addresses in the JNI level and convert to IPv4 addresses before pass to InetAddress.getByName(...) (via InetSocketAddress constructor).

Result:

Eliminate performance problem causes by exception creation when parsing IPv4-mapped-on-IPv6 addresses.
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.