async_wrap/timers: remove Add/RemoveAsyncListener

The ability to add/remove an AsyncListener to an object after its
creation was an artifact of trying to get AL working with the domain
module. Now that is no longer necessary and other features are going to
be implemented that would be affected by this functionality. So the code
will be removed for now to simplify the implementation process.

In the future this code will likely be reintroduced, but after some
other more important matters have been addressed.

None of this functionality was documented, as is was meant specifically
for domain specific implementation work arounds.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
node: ensure same AL inst only runs once

It was possible that the same AL instance was run twice if it were both
attached to the currentContext then again added to the new asyncQueue
generated for the new stack.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
node: compare AsyncListener instances, not uid's

Now that process.createAsyncListener() returns a unique object instance
it is no longer necessary to compare the uid's of the objects.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>