MEDIUM: connection: update callers of ctrl->drain() to use conn_drain()

MEDIUM: connection: update callers of ctrl->drain() to use conn_drain()

Now we can more safely rely on the connection state to decide how to
drain and what to do when data are drained. Callers don't need to
manipulate the file descriptor's state anymore.

Note that it also removes the need for the fix ea90063 ("BUG/MEDIUM:
stream-int: fix the keep-alive idle connection handler") since conn_drain()
correctly sets the polling flags.