Optimise the storage that we use for storing MVCC information.

Writing the memstoreTS information is critical to ensuring Atomic semantics. However, it adds a storage overhead to the disk files. This information is useful only when there is a "concurrent" read operation that needs to ignore a particular key-value. Once it is known that a key-value is going to be included in all current and future read operations, MVCC memstoreTS information is of no use anymore. We can optimize for storage on disk, by getting rid of these values during later compactions.