Clean up in ComputeLogManager on instance cleanup
Best I can tell from https://buildkite.com/dagster/dagster/builds/14878#ddfa5e35-46ae-40f9-99d0-222a1dfb203b
is that there's an issue during teardown:
"Unhandled exception in thread started by <bound method PollingObserver.__bootstrap of <PollingObserver(Thread-82, stopped daemon 139963773277952)>>"
This is an attempt to address that by adding a stop() call to the cleanup path.
Test Plan: BK and pray
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D4856