zebra: Reorganize NHT code

zebra: Reorganize NHT code

NextHop Tracking (NHT) is a significant function introduced into Quagga
by Cumulus. Initially intended for tracking BGP nexthops, this has been
extended subsequently to also cater to nexthops for static routes, BGP
peer reachability tracking and BGP route tracking for routes to be
imported into BGP.

This patch reorganizes the code a bit to make it easier to follow and
maintain. No functional changes introduced.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Dinesh Dutt <ddutt@cumulusnetworks.com>
Ticket: CM-5599
Reviewed By: CCR-3906
Testing Done: Manual and bgpsmoke