When unexpected events are received, do not silently transition to Idle

When unexpected events are received, do not silently transition to Idle
state through bgp_ignore() as that may not do required cleanup. Instead,
define a new event handler to handle such cases, which will go through
bgp_stop(). A similar change is also done to handle the case where an
event handler fails.

Also add a couple of variables to keep track of events for a peer.