[MINOR] listeners: add support for queueing resource limited listeners

[MINOR] listeners: add support for queueing resource limited listeners

When a listeners encounters a resource shortage, it currently stops until
one re-enables it. This is far from being perfect as it does not yet handle
the case where the single connection from the listener is rejected (eg: the
stats page).

Now we'll have a special status for resource limited listeners and we'll
queue them into one or multiple lists. That way, each time we have to stop
a listener because of a resource shortage, we can enqueue it and change its
state, so that it is dequeued once more resources are available.

This patch currently does not change any existing behaviour, it only adds
the basic building blocks for doing that.