I need this to implement SMTP pipelining checks. I mostly need to
flush my send buffer when the read buffer is empty before waiting for
the next command.
Use a counter to count notifications. This protects against spurious
wakeups by pthreads and other libraries. The state transitions now
track num_idle precisely.
Currently, when threads in the blocking pool shutdown due to being idle
the counter tracking threads is not decremented. This prevents new threads
from being spawned to replace the shutdown threads.
Jonathan Bastien-Filiatrault