Cleanup some code related to NHT.

Cleanup some code related to NHT.

When BGP connection setup was moved to rely on nexthop tracking, a few silly
bugs were introduced.
 - bgp_connect_check() was called unnecessarily which resulted in false
   positives which resulted in log messages indicating an error and the FSM
   was unnecessarily reset.
 - When routes to peer disappeared, and the peer was not directly connected,
   the session was not immediately torn down, but only on hold timer expiry.
 - When NHT indicated that route to session IP addr was available, the previous
   state was not reset and as a result, connect retry timer had to expire
   before a reconnection was attempted.
 - connected check MUST be enabled only for EBGP non-multihop sessions and
   only if disbale-connected-check option is not enabled.

This patch fixes all these errors.