BGP: Fix route refresh behavior

BGP: Fix route refresh behavior

On import filter reconfiguration, the route refresh capability is now
honored and used instead of restarting the session.

On export filter reconfiguration, the enhanced route refresh capability
is used to indicate BoRR and EoRR, unless the export table is on. In
such cases, only relevant changes are sent as proper updates.

When route refresh request is received, the enhanced route refresh
capability is now honored.

Co-Authored-By: Maria Matejka <mq@ucw.cz>