acl-plugin: optimize session idle timer checks

This commit adds explicit signaling from a non-owning thread to the owning thread
to restart the session timer as necessary.

Consequently, we now can sweep the session lists at their respective timeouts,
rather than sweeping all the lists at the pace of the shortest timeout value,
just taking care to wake up if the session requeue to a different list results
in needing to wake up earlier.

Change-Id: Ifc8c500f6988748f4cd3dc184dd7824321aaaaca
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>