CommentStore: Accept SCHEMA_COMPAT_* constants

CommentStore: Accept SCHEMA_COMPAT_* constants

Extensions doing an actor and comment migration at the same time will
likely want to use the same constants for each, and that will most
likely be using write-both/read-old and write-both/read-new for the
middle stages rather than write-both/read-both and write-new/read-both
as implemented by the MIGRATION_* constants. See Ie29fd05 for example.

This patch changes CommentStore's internal logic to work correctly when
passed write-both/read-old or write-both/read-new.

Bug: T233449
Change-Id: Iebec80f969ad1c4f9f51a4f25656319cca32b0dd