When a peer that is Established goes down, it is moved into the Clearing

When a peer that is Established goes down, it is moved into the Clearing
state to facilitate clearing of the routes received from the peer - remove
from the RIB, reselect best path, update/delete from Zebra and to other
peers etc. At the end of this, a Clearing_Completed event is generated to
the FSM which will allow the peer to move out of Clearing to Idle.

The issue in the code is that there is a possibility of multiple Clearing
Completed events being generated for a peer, one per AFI/SAFI. Upon the
first such event, the peer would move to Idle. If other events happened
(e.g., new connection got established) before the last Clearing_Completed
event is received, bad things can happen.

Fix to ensure only one Clearing_Completed event is generated.