WorkQueue: Taking care of potential race condition during pause()

WorkQueue: Taking care of potential race condition during pause()
Introduced two variables to keep track of number of threads paused
and drained during threadpool pause/drain. The pause()/drain() call
is waiting till number of pause/drain threads equals to toral number of
thread pool threads.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>