lib: add mt-safe thread_cancel

lib: add mt-safe thread_cancel

This patch implements an MT-safe version of thread_cancel() in
thread_cancel_async(). Behavior as follows:

* Cancellation requests are queued into a list

* Cancellation requests made from the same pthread as the thread_master
  owner are serviced immediately (thread_cancel())

* Cancellation requests made from a separate pthread are queued and the
  call blocks on a condition variable until the owning pthread services
  the request, at which point the condition variable is signaled and
  execution continues (thread_cancel_async())

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