Use an atomic long for SRP counter

Performance tests show that the listener monitor is heavily contended.
We start off by making the SRP counter atomic, so we can reference it
without the lock being held. Also add annotations to the base class, so
we know which fields are protected.

Change-Id: Ie5d0685f766de83fae0d1591a971727b610fc8ea
Signed-off-by: Robert Varga <rovarga@cisco.com>