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
