developer: consolidiate gossip timing options into one --dev-fast-gossip.

It's generally clearer to have simple hardcoded numbers with an
#if DEVELOPER around it, than apparent variables which aren't, really.

Interestingly, our pruning test was always kinda broken: we have to pass
two cycles, since l2 will refresh the channel once to avoid pruning.

Do the more obvious thing, and cut the network in half and check that
l1 and l3 time out.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/opening_control.c

@@ -949,7 +949,8 @@ void peer_start_openingd(struct peer *peer,
 				  peer->localfeatures,
 				  local_feature_negotiated(peer->localfeatures,
 							   LOCAL_STATIC_REMOTEKEY),
-				  send_msg);
+				  send_msg,
+				  IFDEV(peer->ld->dev_fast_gossip, false));
 	subd_send_msg(uc->openingd, take(msg));
 }