flat_mutation_reader_assertions: add fast_forward_to(position_range)
flat_mutation_reader_assertions: add fast_forward_to(position_range)

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <7b530909cf188887377aec3985f9f8c0e3b9b1e8.1511180262.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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