BGP: Fix nexthop registration churn

BGP: Fix nexthop registration churn

When a BGP nexthop is registered for resolution, if it is learnt from an
EBGP peer and other conditions warrant (non-multihop peer and connected check
is not disabled), the registration includes a flag that indicates that the
nexthop must be resolved only if it is directly connected. In peculiar
situations - e.g., third-party nexthop or policy configuration - the same
nexthop could be learnt from an IBGP peer, and in general, nexthops learnt
from IBGP peers can be resolved over any route. This scenario was causing
a churn in the nexthop registration with the 'must-be-connected' flag being
repeatedly toggled as routes are received from both peers. The registrations
would in turn trigger significant processing.

The fix is to treat 'must-be-connected' as an overriding condition.

The repeated registration and related processing was also causing heavy memory
usage by BGP - for memory buffers used to hold registration information. This
fix will ensure that is no longer the case.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-8005, CM-8013
Reviewed By: CCR-3772
Testing Done: Manual, bgpsmoke (on 2.5-br)