BUG/MINOR: listener: Wake proxy's mngmt task up if necessary on session release

BUG/MINOR: listener: Wake proxy's mngmt task up if necessary on session release

When a session is released, listener_release() function is called to notify
the listener. It is an opportunity to resume limited/full listeners. We
first try to resume the listener owning the released session, then all
limited listeners in the global queue and finally all limited listeners in
the frontend's waiting queue. This last step is only performed if there is
no limit applied on the frontend. Nothing is performed if the session rate is
still limited. And it is an issue because if this happens for the last
listener's session, there is no other event to wake the frontend's managment
task up and the listener remains in the limited state.

To fix the issue, when a limit is still applied on the frontent, we must
compute the new wake up date from the sessions rate and schedule the
frontend's managment task.

It is easy to reproduce the issue in SSL by setting a maxconn and a rate
limit on sessions.

This patch should fix the issue #2476. It must be backported to all stable
versions.