async-wrap: add event hooks

Call a user-defined callback at specific points in the lifetime of an
asynchronous event. Which are on instantiation, just before/after the
callback has been run.

**If any of these callbacks throws an exception, there is no forgiveness
or recovery. A message will be displayed and a core file dumped.**

Currently these only tie into AsyncWrap, meaning no call to a hook
callback will be made for timers or process.nextTick() events. Though
those will be added in a future commit.

Here are a few notes on how to make the hooks work:

- The "this" of all event hook callbacks is the request object.

- The zero field (kCallInitHook) of the flags object passed to
  setupHooks() must be set != 0 before the init callback will be called.

- kCallInitHook only affects the calling of the init callback. If the
  request object has been run through the create callback it will always
  run the before/after callbacks. Regardless of kCallInitHook.

- In the init callback the property "_asyncQueue" must be attached to
  the request object. e.g.

  function initHook() {
    this._asyncQueue = {};
  }

- DO NOT inspect the properties of the object in the init callback.
  Since the object is in the middle of being instantiated there are some
  cases when a getter is not complete, and doing so will cause Node to
  crash.

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>