Exit main thread if a worker thread terminates

Terminate the main thread if a worker thread terminates.

Also:

• add names to the worker threads to make them more easily
  identifiable in stack traces.

• don't dump a stack trace if the user control-C's the program.

Bug: T351478

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Related MRs:

• !25 (merged) (This MR)
• !24 (merged)
• !23 (merged)
• main