Persist memstoreTS to disk

Atomicity can be achieved in two ways – by using a multiversion concurrency system (MVCC), or (ii) by ensuring that "new" writes do not complete, until the "old" reads complete.

Currently, Memstore uses something along the lines of MVCC (called RWCC for read-write-consistency-control). But, this mechanism is not incorporated for the key-values written to the disk, as they do not include the memstore TS.

Let us make the two approaches be similar, by persisting the memstoreTS along with the key-value when it is written to the disk.