Thread pool: Patch previous commit

Thread pool: Patch previous commit

I accidentally removed a lock operation in the previous commit,
so lots of undefined behavior was being triggered.

Also, restores (but improves) the thread ready signal. It's hard to
explain:

- Before: Workers send ready signal to parent,
  but parent might not be listening yet;
  Therefore parent timeouts on wait.
- Previous: Workers do not send ready signal to parent.
  Therefore, parent might signal work when no workers are ready yet;
  Therefore nobody works.
- Now: Workers send ready signal to parent,
  parent listens lazily (ie. late), but only if workers aren't ready
  yet.
  Therefore, correct behavior.