manager: Use separate lock for session event notification.

When notifying a manager session that new events were available
the same lock was used that was also held when doing things within
the session (such as sending events out). If the manager session
blocked for a period of time this would cause a back up of messages
in Stasis and would also block any other sessions from receiving
events.

This change adds a separate lock to the manager session which is
strictly used for notifying it that new events are available.

ASTERISK-28350

Change-Id: Ifbcac007faca9ad0231640f5e82a6ca9228f261b