Changes to improve BGP convergence time:

Changes to improve BGP convergence time:

- Schedule write thread for advertisements and withdraws only if corresponding
  FIFOs are growing and/or upon work_queue getting fully processed.
- Set non-default yield time for the main work_queue, as the default value
  of 10ms results in yielding after processing very few nodes.
- Remove unnecessary scheduling of write thread when update packet is formed.
- If MRAI is 0, don't start a timer unnecessarily, directly schedule write
  thread.
- Some debugs.