Fix BGPPeer internal locking

Fix BGPPeer internal locking

onRouteRefreshMessage() is not synchronized and manipulates
adjRibOutListenerSet, which is a thread-safety hazard.

Fix by holding a lock for the duration needed, as well as
use a single remove() operation instead of get/remove combo.

Finally lower private method synchronization and replace it
with @GuardedBy annotations, so that it is understood those
methods are only ever called with the lock already held.

Change-Id: Idd509a345dfc3afa800e7456fb6d3bdbd1f367e7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>