To ensure isolation of operation when streaming a mutation from a
mutable source (such as cache or memtable) MVCC is used.
Each entry in memtable or cache is actually a list of used versions of
that entry. Incoming writes are either applied directly to the last
verion (if it wasn't being read by anyone) or preprended to the list
(if the former head was being read by someone). When reader finishes it
tries to squash versions together provided there is no other reader that
could prevent this.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
