Lock thread_pool.wait_mutex before forking to avoid a race condition between rad_fork, rad_waitpid and reap_children.

There is a race condition that can occur under high load where a child is reaped before being added to the waiters list.
Merge pull request #824 from jrouzierinverse/v2.x.x

Lock thread_pool.wait_mutex before forking to avoid a race condition bet...