Now that mod_mam no longer uses the 'offline_message_hook', avoid
duplicating notifications for messages written to both MAM and offline
storage in another way.
When a client enabled push notifications during the current session,
notifications should be suppressed as long as the client is online.
Suppressing the notification didn't work for the case where the
notification was triggered by MAM, but this is now fixed.
This module tries to keep pending stream management sessions of push
clients alive (as long as the disconnected clients are reachable via
push notifications).
