Introduce `ms` -- a new sstable format version which
is a hybrid of Cassandra's `me` and `da`.
It is based on `me`, but with the index components
(Summary.db and Index.db) replaced with the index
components of `da` (Partitions.db and Rows.db).
As of this patch, the version is never chosen
anywhere for writing sstables yet. It is only introduced.
We will add it to unit tests in a later commit,
and expose it to users in yet later commit.
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