zebra: some more i/o optimizations

zebra: some more i/o optimizations

* Separate flush task from write task, so we can continue adding to the
  write buffer while it's waiting to flush
* Handle write errors sooner rather than later
* Only schedule a process job if we have packets to process
* Tweak zserv_process_messages to not reschedule itself and rely on
  zserv_read() to do so in all proper cases

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>