Manage log locks on the Log object to prepare for multiple writers

Summary:
Depends on D19134. Ref T13088. Future changes will support API writers, so push the log lock into the Log object.

Allow open/close ("this process is writing to this log") to be separate from live/final ("this log is still generating more data").

Test Plan: Wrote logs with bin/harbormater write-log and updated logs with bin/harbormaster rebuild-log.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19135