src: remove Async Listener

Async Listener was the name of the user-facing JS API, and is being
completely removed. Instead low level hooks directly into the mechanism
that AL used will be introduced in a future commit.

PR-URL: https://github.com/joyent/node/pull/8110


Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Alexis Campailla <alexis@janeasystems.com>
Reviewed-by: Julien Gilli <julien.gilli@joyent.com>