Autogen: Redo locking and state machine for fence handling and the worker pool.

(1) All CV use must hold the corresponding mutex, otherwise race conditions happen. This is mandated by the C++ standard.

(2) Introduce a separate CV for the thread waiting for other jobs to finish before running a fence. This avoids waking up all other workers blindly. Correctly wake that thread up when the processing of outstanding jobs is done.

(3) Split the waiting for a fence to become runnable from a fence is pending. This avoids problems if more than one fence can end up on the queue. The thread that took a fence off the queue is responsible for clearing the fence processing flag.

Topic-rename: autogen-fix-workerpool-locking