workerpool: Allow processing of new tasks to be stopped

This is different from “Quiesce” in the sense that this function just
changes an internal flag and doesn't wait for the queue to be empty.
Tasks already being processed continue normally, but no new tasks will
be started. New tasks can still be added, but won't be processed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>