Update the Javadoc of ChannelHandler and ChannelHandlerContext

Motivation:

It's out of date and it has broken links, etc.

Modifications:

Backport the fixes from master (132af3a485015ff912bd567a47881814d2ce1828)

Result:

Better Javadoc
Correct the return type of MultithreadEventLoopGroup.newChild()

Motivation:

MultithreadEventLoopGroup.newChild() does not override MultithreadEventExecutorGroup.newChild() which returns EventExecutor.  MultithreadEventLoopGroup.newChild() should never return an EventExecutor, so this is incorrect.

Modifications:

Override MultithreadEventLoopGroup.newChild() so that it returns EventLoop

Result:

Correct API
User-definable thread model via ChannelHandlerInvoker

Motivation:

While the default thread model provided by Netty is reasonable enough for most applications, some users might have a special requirement for the thread model.  Here are a few examples:

- A user might want to invoke handlers from the caller thread directly, assuming that his or her application is completely asynchronous and does not make any invocation from non-I/O thread.  In this case, the default invoker implementation will only add the overhead of checking if the current thread is an I/O thread or not.
- A user might want to invoke handlers from different threads depending on the type of events flexibly.

Modifications:

- Backport 132af3a485015ff912bd567a47881814d2ce1828 which is a fix for #1912
  - Add a new interface called 'ChannelHandlerInvoker' that performs the invocation of event handler methods.
  - Add pipeline manipulation methods that accept ChannelHandlerInvoker
- The differences from the original commit:
  - Separated the irrelevant changes out
  - Channel.eventLoop is null until the registration is complete in this branch, so Channel.Unsafe.invoker() doesn't work before registration.
  - Deregistration is not gone in this branch, so the methods related with deregistration were added to ChannelHandlerInvoker