[bgpd] reference count the BGP instance

[bgpd] reference count the BGP instance

When a BGP instance is deleted with lots of routes and neighbors
it is possible for the peer rsclient queue to run after
bgp_delete has been called. This would lead to bgpd crashing,
see https://bugzilla.vyatta.com/show_bug.cgi?id=3436

The fix is to add reference counting to the BGP instance and defer
actual freeing until all references are gone.

This patch also fixes a memory leak where the self-reference
peer instance was being created but never freed.

The check in bgp_clear_route is no longer valid because it is possible
for it to be called when peer is in Deleted state during cleanup.