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>

tests/test_closing.py

@@ -29,7 +29,7 @@ def test_closing(node_factory, bitcoind):
 
     # Only wait for the channels to activate with DEVELOPER=1,
     # otherwise it's going to take too long because of the missing
-    # --dev-broadcast-interval
+    # --dev-fast-gossip
     if DEVELOPER:
         wait_for(lambda: len(l1.getactivechannels()) == 2)
         wait_for(lambda: len(l2.getactivechannels()) == 2)