Move Harbormaster log compression to the worker task queue

Summary: 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).

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

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19131