Clarify concurrent.futures requirement error message

Specify which PyPI package needs to be installed in order for the gthread
worker implementation to work on Python < 3.2.
send SIGQUIT/SIGTERM only once to the workers

there is no need to send more than once the signal to the workers when
exiting. With this patch we are sending once the QUIT/TERM signal and  then
wait until the graceful timeout or until there are workers alive.

fix #655
fix run loop in sync worker
remove dynamic timeout temporarely

this change is removing the dynamic timeout to make it sure it doesn't have
any side effect in the worker monitored like reported in #942