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".

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