zebra: fix some memory errors, scheduling bugs

zebra: fix some memory errors, scheduling bugs

* Add doc comments explaining hairy bits of thread lifecycle
* Remove t_suicide as it no longer makes sense
* Remove client double-free
* Remove unnecessary THREAD_OFF being used in incorrect pthread context
* Eliminate unnecessary racey access to client's obuf_fifo
* Ensure zserv_process_messages() reschedules itself if it has not
  finished its work

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