stream_manager: Stop instances on drain
stream_manager: Stop instances on drain

It's not seen directly from ths patch itself, but the only
difference between first several calls that drain() makes
and the stop_transport() is the do_stop_stream_manager()
in the latter.

Again, it's partially a bugfix (shutdown after drain leaves
streaming running), partially a must-have thing (streaming
is not expected in the background after drain), partially a
unification of two drains out there.

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

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