[#2656] Minimize ByteBuf.writeBytes(...) calls by pack the separator into the HttpHeaderEntity.

Motivation:
Currently we do 4 ByteBuf.writeBytes(...) calls per header line. This is can be improved.

Modification:
Introduce two new HttpHeaders methods to allow create HttpHeaderEntity which contains the separator. With this we can minimize it to 2 ByteBuf.writeBytes(...) calls per header line

Result:
Performance improvement.
Fixed NPE in WebSocket00FrameDecoder if end couldn't be found in text frame

Motivation:
When we receive an incomplete WebSocketFrame we need to make sure to wait for more data. Because we not did this we could produce a NPE.

Modification:
Make sure we not try to add null into the RecyclableArrayList

Result:
no more NPE on incomplete frames.
[#2664] Support write-spinning also in native transport

Motivation:

In our nio implementation we use write-spinning for maximize throughput, but in the native implementation this is not used.

Modification:

Respect writeSpinCount in native transport.

Result:

Better throughput
 [#2647] Handle IOV_MAX in java code

Motivation:

The handling of IOV_MAX was done in JNI code base which makes stuff really complicated to maintain etc.

Modifications:

Move handling of IOV_MAX to java code to simplify stuff

Result:

Cleaner code.
[#2666] Fix possible NPE when try to fullfill connect ChannelPromise

Motivation:

Because of a missing return statement we may produce a NPE when try to fullfill the connect ChannelPromise when it was fullfilled before.

Modification:

Add missing return statement.

Result:

No more NPE.
[#2667] Write until EAGAIN in native transport

Motivation:

We need to continue write until we hit EAGAIN to make sure we not see an starvation

Modification:

Write until EAGAIN is returned

Result:

No starvation when using native transport with ET.
[#2665] Continue writing on IOException when using DatagramChannel

Motivation:

As a DatagramChannel supports to write to multiple remote peers we must not close the Channel once a IOException accours as this error may be only valid for one remote peer.

Modification:

Continue writing on IOException.

Result:

DatagramChannel can be used even after an IOException accours during writing.
[#2667] Write until EAGAIN in native transport and only call setEpollOut() in this case

Motivation:

In the previous fix for #2667 I did introduce a bit overhead by calling setEpollOut() too often.

Modification:

Only call setEpollOut() if really needed and remove unused code.

Result:

Less overhead when saturate network.
Close ObjectInputStream in ObjectDecoder.decode(...)

Motivation:

We create a new CompactObjectInputStream with ByteBufInputStream in ObjectDecoder.decode(...) method and don't close this InputStreams before return statement.

Modifications:

Save link to the ObjectInputStream and close it before return statement.

Result:

Close InputStreams and clean up unused resources. It will be better for GC.
Small performance improvements

Modifications:

- Added a static modifier for CompositeByteBuf.Component.
This class is an inner class, but does not use its embedded reference to the object which created it. This reference makes the instances of the class larger, and may keep the reference to the creator object alive longer than necessary.
A boxed primitive is created from a String, just to extract the unboxed primitive value.
- Removed unnecessary checks if file exists before call mkdirs() in NativeLibraryLoader and PlatformDependent.
Because the method mkdirs() has this check inside.

Conflicts:
	codec-http/src/main/java/io/netty/handler/codec/http/multipart/DiskAttribute.java
	codec-stomp/src/main/java/io/netty/handler/codec/stomp/StompSubframeAggregator.java
	codec-stomp/src/main/java/io/netty/handler/codec/stomp/StompSubframeDecoder.java
Fixes for compression codecs

Motivation:

Fixed founded mistakes in compression codecs.

Modifications:

- Changed return type of ZlibUtil.inflaterException() from CompressionException to DecompressionException
- Updated @throws in javadoc of JZlibDecoder to throw DecompressionException instead of CompressionException
- Fixed JdkZlibDecoder to throw DecompressionException instead of CompressionException
- Removed unnecessary empty lines in JdkZlibEncoder and JZlibEncoder
- Removed public modifier from Snappy class
- Added MAX_UNCOMPRESSED_DATA_SIZE constant in SnappyFramedDecoder
- Used in.readableBytes() instead of (in.writerIndex() - in.readerIndex()) in SnappyFramedDecoder
- Added private modifier for enum ChunkType in SnappyFramedDecoder

Result:

Fixed sum overflow in Bzip2HuffmanAllocator, improved exceptions in ZlibDecoder implementations, hid Snappy class
Small fixes and improvements

Motivation:

Fix some typos in Netty.

Modifications:

- Fix potentially dangerous use of non-short-circuit logic in Recycler.transfer(Stack<?>).
- Removed double 'the the' in javadoc of EmbeddedChannel.
- Write to log an exception message if we can not get SOMAXCONN in the NetUtil's static block.
Fix NPE problems

Motivation:

Now Netty has a few problems with null values.

Modifications:

- Check File in DiskFileUpload.toString().
If File is null we will get NPE when calling toString() method.
- Check Result<String> in MqttDecoder.decodeConnectionPayload(...).
- Check Unsafe before calling unsafe.getClass() in PlatformDependent0 static block.
- Removed unnecessary null check in WebSocket08FrameEncoder.encode(...).
Because msg.content() can not return null.
- Removed unnecessary null checks in ConcurrentHashMapV8.removeTreeNode(TreeNode<K,V>).
- Removed unnecessary null check in OioDatagramChannel.doReadMessages(List<Object>).
Because tmpPacket.getSocketAddress() always returns new SocketAddress instance.
- Removed unnecessary null check in OioServerSocketChannel.doReadMessages(List<Object>).
Because socket.accept() always returns new Socket instance.
- Pass Unpooled.buffer(0) instead of null inside CloseWebSocketFrame(boolean, int) constructor.
If we will pass null we will get NPE in super class constructor.
- Added throw new IllegalStateException in GlobalEventExecutor.awaitInactivity(long, TimeUnit) if it will be called before GlobalEventExecutor.execute(Runnable).
Because now we will get NPE. IllegalStateException will be better in this case.
- Fixed null check in OpenSslServerContext.setTicketKeys(byte[]).
Now we throw new NPE if byte[] is not null.

Result:

Added new null checks when it is necessary, removed unnecessary null checks and fixed some NPE problems.
Rewrite HttpObjectDecoder to use ByteToMessageDecoder

Motivation:
HttpObjectDecoders uses ReplayingDecoder which is slow compared to ByteToMessageDecoder because it needs a lot more range checks and also throws a Signal very often.

Modifications:
 - Rewrite HttpObjectDecoder to use ByteToMessageDecoder
 - Not parse headers multiple times
 - Simplify the code

Result:
Performance improvements.