memtable: add support for fast_forward_to()
memtable: add support for fast_forward_to()

Fast forwarding of memtable readers is needed only for unit tests which
often use memtables as underlying data source for cache and the cache
readers.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla