Fix a potential deadlock in BGPSession

Fix a potential deadlock in BGPSession

In a two-session scenario, it is possible to arrive at a ABBA deadlock,
where we acquire locks:

Thread 1      Thread 2
--------      --------
Session A     Session B

RIB
              RIB <-- blocks waiting for Thread 1

Session B <-- blocks waiting for Thread 2

The lock is being held only for scheduling a task on Thread 2, which a
thread-safe operation, so we can simply remove the need to take the lock
in Thread 1.

Change-Id: I4e55d3228a48790b2f19eef12ce1b54fd57955a0
Signed-off-by: Robert Varga <rovarga@cisco.com>