WorkerThreadPool: Fix end-of-yield logic potentially leading to deadlocks

[RandomShaper]

Without this, certain situations with much thread interactions can deadlock. The specific case I run into was multi-threaded resource loading from the editor with progress dialog running iterations and also separate-threaded physics.

If the flag was unconditionanlly reset, this could happen:

• A worker thread (A) is used by a server.
• Thread A starts yielding.
• Thread A is awaken to start processing a task.
• Thrad B triggers end-of-yield on thread A so the relevant flag is set.
• Thread A ends processing its current task and clears the flag.
• Thread A goes back to yielding without a chance of seeing the flag set so it keeps waiting forever.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.3.1.