lib: mt-safe cancel, round deux

lib: mt-safe cancel, round deux

* Update pollfds copy as well as the original
* Keep array count for copy in thread_master
* Remove last remnants of POLLHUP in .events field
* Remove unused snmpcount (lolwut)
* Improve docs
* Add missing do_thread_cancel() call in thread_cancel_event()
* Change thread_fetch() to always enter poll() to avoid starving i/o
* Remember to free up cancel_req when destroying thread_master
* Fix dereference of null pointer
* Fix dead store to timeval
* Fix missing condition for condition variable :-)

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