Make Nio/EpollEventLoop run on a ForkJoinPool

Related issue: #2250

Motivation:

Prior to this commit, Netty's non blocking EventLoops
were each assigned a fixed thread by which all of the
EventLoop's I/O and handler logic would be performed.

While this is a fine approach for most users of Netty,
some advanced users require more flexibility in
scheduling the EventLoops.

Modifications:

Remove all direct usages of threads in MultithreadEventExecutorGroup,
SingleThreadEventExecutor et al., and introduce an Executor
abstraction instead.

The way to think about this change is, that each
iteration of an eventloop is now a task that gets scheduled
in a ForkJoinPool.

While the ForkJoinPool is the default, one also has the
ability to plug in his/her own Executor (aka thread pool)
into a EventLoop(Group).

Result:

Netty hands off thread management to a ForkJoinPool by default.
Users can also provide their own thread pool implementation and
get some control over scheduling Netty's EventLoops
Deregistration of a Channel from an EventLoop

Motivation:

After a channel is created it's usually assigned to an
EventLoop. During the lifetime of a Channel the
EventLoop is then responsible for processing all I/O
and compute tasks of the Channel.

For various reasons (e.g. load balancing) a user might
require the ability for a Channel to be assigned to
another EventLoop during its lifetime.

Modifications:

Introduce under the hood changes that ensure that Netty's
thread model is obeyed during and after the deregistration
of a channel.

Ensure that tasks (one time and periodic) are executed by
the right EventLoop at all times.

Result:

A Channel can be deregistered from one and re-registered with
another EventLoop.