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.