ChangeTags: turn private getPrevTags() into public getTags()

ChangeTags: turn private getPrevTags() into public getTags()

This makes the most convenient way to get all the change tags for a
given recent change, revision, and/or log entry public. The database is
injected as an additional parameter so that callers can choose to read
from a replica, and the revision and log ID parameters are swapped for
consistency with other methods in the same class. Also, while we’re
already touching the method, let’s extract the change_tag_def store.

Change-Id: Id8f14044ba5d926447aedd60e8cc8eb7dc864899