Depends on D19130. Ref T13088. Currently, when a build log is closed we compress it in the same process. Separate this out into a dedicated worker since the plan is to do a lot more work during finalization, none of which needs to happen inline during builds (or, particuarly, inline during a Conduit call for API writes in the future).

Ran bin/harbormaster write-log --trace, saw compression run inline.