[#2414] Fix IllegalStateException when try to configure AbstractEpollChannel once it is deregistered

Motivation:
AbstractEpollChannel.clearEpollIn() throws an IllegalStateException if a user tries to change the autoRead configuration for the Channel and the Channel is not registered on an EventLoop yet. This makes it for example impossible to set AUTO_READ to false via the ServerBootstrap as the configuration is modifed before the Channel is registered.

Modification:
Check if the Channel is registered and if not just modify the flags directly so they are respected once the Channel is registered

Result:
It is possible now to configure AUTO_READ via the ServerBootstrap
Make NioSocketChannelTest more bullet-proof

Motivation:
I had the NioSocketChannelTest.testFlushCloseReentrance() fail sometimes on one of my linux installation. This change let it pass all the time.

Modification:
Set the SO_SNDBUF to a small value to force split writes

Result:
Test is passing all the time where it was sometimes fail before.
Mark sun.nio.ch as optional

Motivation:

If sun.nio.ch is not optional this will cause troubles in the
OSGi world. The package is not exposed by default in OSGi, so
actually the whole netty framework cannot be used directly.

There are workarounds, but workarounds are ugly. Especially since
the use of sun.nio.ch is optional. So the requirement on the
package should be optional as well.

Modifications:

Make the import of sun.nio.ch optional.

Result:

If the package cannot be imported it will behave as if the package
sun.nio.ch is not present (like with other JVMs). If the package is
exposed in OSGi (e.g. bootclassloader delegation, extension fragment)
it will be used.
[codec-memcache] Avoid NPE on channelInactive.

Motivation:
When no currentMessage has been set and the channel is inactive, a NPE is raised.

Modification:
Make sure that a currentMessage is available before checking the extras.

Result:
No more NPE raised potentially.
Resurrect channel deregistration and constructor changes

Motivation:

Due to the complexity of handling deregistration and re-registration of
a channel, we previously decided to remove the deregister() operation
completely to simplify our code.  However, we realized that it shouldn't
be that complicated to implement it during our discussion about making
I/O scheduling more flexible and more customizable [1], and thus the
removal of deregistration and re-registration is unnecessary now.

Modification:

- Revert commit c149f4bcc0c0d02aa1abcd5e39c155a9e598822e
- Revert commit e743a27e752f0badaf71688457793d0aa365a3ac
- Make some additional adjustments

Result:

- deregister(), fireChannelUnregistered(), and channelRegistered() were
  added back..
- Channel constructors do not require an EventLoop anymore.

[1] https://github.com/netty/netty/issues/2250
Synchronized between 4.1 and master

Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Fix found differences

Result:

4.1 and master got closer.
Synchronized between 4.1 and master

Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Fix found differences

Result:

4.1 and master got closer.
Remove unused method to match up 4.1 branch

 Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Remove ChannelHandlerInvokerUtil.invokeWriteAndFlush(...) as it is not used anyway and not exists in 4.1

Result:

4.1 and master got closer.
Synchronized between 4.1 and master again (part 2)

Motivation:
4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:
Remove ChannelHandlerInvoker.writeAndFlush(...) and the related
implementations.

Result:
4.1 and master got closer.
Fix compilation errors
Synchronized between 4.1 and master (part 3)

Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Fix found differences

Result:

4.1 and master got closer.
Undeprecate deregister() and chanelUnregistered()

Motivation:

As discussed in #2250, it will become much less complicated to implement
deregistration and reregistration of a channel once #2250 is resolved.
Therefore, there's no need to deprecate deregister() and
channelUnregistered().

Modification:

- Undeprecate deregister() and channelUnregistered()
- Remove SuppressWarnings annotations where applicable

Result:

We (including @jakobbuchgraber) are now ready to play with #2250 at
master
Code clean-up

Motivation:

It is less confusing not to spread Thread.interrupt() calls.

Modification:

- Comments
- Move generatorThread.interrupt() to where currentThread.interrupt() is
  triggered

Result:

Code that is easier to read
Eliminate unnecessary extra synchronization in DefaultChannelPipeline

Motivation:
At the moment whenever we add/remove a ChannelHandler with an EventExecutorGroup we have two synchronization points in the execution path. One to find the childInvoker and one for add/remove itself. We can eliminate the former by call findIInvoker in the synchronization block, as we need to synchronize anyway.

Modification:
Remove the usage of AtomicFieldUpdater and the extra synchronization in findInvoker by moving the call of the method in the synchronized(this) block.

Result:
Less synchronization points and volatile reads/writes
Adding support for echoing the request origin for CORS.

Motivation:
When CORS has been configured to allow "*" origin, and at the same time
is allowing credentials/cookies, this causes an error from the browser
because when the response 'Access-Control-Allow-Credentials' header
is true, the 'Access-Control-Allow-Origin' must be an actual origin.

Modifications:
Changed CorsHandler setOrigin method to check for the combination of "*"
origin and allowCredentials, and if the check matches echo the CORS
request's 'Origin' value.

Result:
This addition enables the echoing of the request 'Origin' value as the
'Access-Control-Allow-Origin' value when the server has been configured
to allow any origin in combination with allowCredentials.

This allows client requests to succeed when expecting the server to
be able to handle "*" origin and at the same time be able to send cookies
by setting 'xhr.withCredentials=true'. A concrete example of this is
the SockJS protocol which expects behaviour.