Simplify the ManagedEventLoopGroup, renaming it to simply EventLoop.

It has the following features:

  - getEventLoopGroup() for populating Netty thingies. Zero
    of the threads participating in the ELG are the "primary"
    user thread. These threads are *purely* for usage by Netty
    stuff that DOES NOT BLOCK.  Since we still leverage the
    Vertx event-loop, this ELG actually should have 2*CPUs
    threads in it.  On my machine, that's 16 threads.

  - submitUserTask(runnable) for submitting a task to execute
    on the "primary" thread. All callbacks into user .js code
    should occur within the user-task executor.  This also
    including the initial running of node.js

Also, accomodating these changes is some jostling around
AsyncWrap.  AsyncWrap's emit() now overrides the bare
EventSource emit() so as to toss the request to emit
into the user-task executor.  If you need direct
emittance without that magic, just use EventSource directly.
Currently on HTTPParser has this requirement, as all other
event-emitting things are ultimately children of AsyncWrap.

Basically, if you are emitting events from the Netty pipeline,
we *must* toss them back to the user-tasks to get them into the
correct thread in the .js side of the world. All current event-emitting
Netty handlers take an AsyncWrap (no longer just an EvenSource)
to ensure we're doing that.

Additionally, our Blocking helper has been adjusted so that unblock()
also forces a throw into the user-task executor.