bgpd: bgp process queue optimization

bgpd: bgp process queue optimization

There are several code paths that dump nodes to the queue for route
processing in a loop. This patch tries to reduce memory allocations/freeing
(work item, list node) and thread scheduling overhead by batching the nodes
in a simple queue list.

In the past when route processing wasn't event driven (bgp_scan()), this used
to have a noticeable impact in table loading, convergence time and memory heap
fragmentation due to reduced alloc's/free's.

Signed-off-by: Jorge Boncompte <jbonor@gmail.com>