Store the Harbormaster log chunk format on the log record

Summary: Depends on D19137. Ref T13088. This allows rebuild-log to skip work if the chunks are already compressed. It also prepares for a future GC which is looking for "text" or "gzip" chunks to throw away in favor of archival into Files; such a GC can use this column to find collectable logs and then write "file" to it, meaning "chunks are gone, this data is only available in Files".

Test Plan: Ran migration, saw logs populate as "text". Ran rebuild-log, saw logs rebuild as "gzip".

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19138