stream_transfer_task: Get manager from session (result-future)
stream_transfer_task: Get manager from session (result-future)

When the task starts it needs the stream_manager to get messaging
service and database from. There's a session at hands and this
session is properly initialized thus it has the result-future.
Voila -- we have the manager!

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla