Fix space leak when pipeline is closed
Add a new control message to drop remove worklets.
Implement `Drop` for `Worklet` and get the worklet thread pool. Use that
pool to send `ExitWorklet` to all the threads with the id of the
`Worklet` that it's dropping.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#17442
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I don't know how to add a test for this :(
If you consider this needs a test, could you point me out on how to write (and run!) one in this project? I gave this a couple tries, but I wasn't very successful.
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