- Protect worker manager with a mutex. - Terminate thread workers which are started after stop was requested + test.
- Protect worker manager with a mutex.
- Terminate thread workers which are started after stop was requested + test.

PiperOrigin-RevId: 384878522
Change-Id: I102ab046a76a8e093affa6a98f4cc6f9ee2d0763

deepmind/launchpad,deepmind/launchpad,deepmind/launchpad,deepmind/launchpad