[#2375] [#2404] Fix bug in respecting ChannelConfig.setAutoRead(false) and also fix Channel.read() for OIO

Motivation:
At the moment ChanneConfig.setAutoRead(false) only is guaranteer to not have an extra channelRead(...) triggered when used from within the channelRead(...) or channelReadComplete(...) method. This is not the correct behaviour as it should also work from other methods that are triggered from within the EventLoop. For example a valid use case is to have it called from within a ChannelFutureListener, which currently not work as expected.

Beside this there is another bug which is kind of related. Currently Channel.read() will not work as expected for OIO as we will stop try to read even if nothing could be read there after one read operation on the socket (when the SO_TIMEOUT kicks in).

Modifications:
Implement the logic the right way for the NIO/OIO/SCTP and native transport, specific to the transport implementation. Also correctly handle Channel.read() for OIO transport by trigger a new read if SO_TIMEOUT was catched.

Result:
It is now also possible to use ChannelConfig.setAutoRead(false) from other methods that are called from within the EventLoop and have direct effect.
[#2405] Add support for SO_REUSEPORT to EpollDatagramChannel

Motivation:
With SO_REUSEPORT it is possible to bind multiple sockets to the same port and so handle the processing of packets via multiple threads. This allows to handle DatagramPackets with more then one thread on the same port and so gives better performance.

Modifications:
Expose EpollDatagramChannelConfig.setReusePort(..) and isReusePort()

Result:
Allow to bind multiple times to the same local address and so archive better performance.
[#2144] Fix NPE in Local transport caused by a race

Motivation:
At the moment it is possible to see a NPE when the LocalSocketChannels doRegister() method is called and the LocalSocketChannels doClose() method is called before the registration was completed.

Modifications:
Make sure we delay the actual close until the registration task was executed.

Result:
No more NPE
[#2401] Improve documentation of HttpObjectAggregator

Motivation:
Make it more clear what the output of HttpObjectAggregator is and that it need to come after the encoder in the pipeline.

Modifications:
Change javadocs to make things more clear.

Result:
Better docs
Move validatePromise(...) to ChannelHandlerInvokerUtil. Related to [#2398]

Motivation:
Once a user implement a custom ChannelHandlerInvoker it is needed to validate the ChannelPromise. We should expose a utility method for this.

Modifications:
Move validatePromise(...) from DefaultChannelHandlerInvoker to ChannelHandlerInvokerUtil and make it public.

Result:
User is able to reuse code
HTTP2 server should not send preface string

Motivation:

See: https://github.com/netty/netty/issues/2402
See: https://tools.ietf.org/html/draft-ietf-httpbis-http2-10#section-3.5

Only the client should send the preface string, the server should not.

"The server connection header consists of just a SETTINGS frame (Section 6.5)
 that MUST be the first frame the server sends in the HTTP/2 connection."

Modifications:

Split out Http2ClientPrefaceWriter and Http2ServerPrefaceReader from
Http2FrameEncoder and Http2FrameDecoder.  The new channel handlers are added
to the pipeline and remove themselves after the preface is written/read.

Result:

HTTP2 client sends preface, server does not, in compliance with spec.