We need to be able to replay mutations created using older versions of
the table's schema. frozen_mutation can be only read using the version
it was serialized with, and there is no guarantee that the node will
know this version at the time of replay. Currently versions kept
in-memory so a node forgets all past versions when it restarts.
To solve this, let's store canonical_mutations which, like data in
sstables, can be read using any later schema version of given table.
ec12b75426