We want to avoid lost messages in the common cases.
This generalizes our drain code, by giving the subds each 5 seconds to
close themselves, but continue to allow them to send us traffic (if
peer is still connected) and continue to send them traffic.
We continue to send traffic *out* to the peer (if it's still
connected), until all subds are gone. We still have a 5 second timer
to close the connection to peer.
On reconnects, we don't do this "drain period" on reconnects: we kill
immediately.
We fix up one test which was looking for the "disconnect" message
explicitly.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>