stream_session: Allow adding ranges to a cf more than once
stream_session: Allow adding ranges to a cf more than once

Append the ranges to a stream_transfer_task if the cf is already added to
_transfers in add_transfer_ranges.

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