Implement DestructibleInterface on BuildLog

Summary: Depends on D19133. Ref T13088. Allows build logs to be formally destroyed, cleaning up their chunks and file data.

Test Plan:

• Used bin/remove destroy to destroy a log, verified chunks and files were removed.
• Used bin/harbormaster rebuild-log to force a log to rebuild, verified files were destroyed and regenerated.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19134