onchaind: fulfill HTLCs onchain.

When we see an offered HTLC onchain, we need to use the preimage if we
know it.  So we dump all the known HTLC preimages at startup, and send
new ones as we discover them.

This doesn't cover preimages we know because we're the final
recipient; that can happen if an HTLC hasn't been irrevocably
committed yet.  We'll do that in a followup patch.

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

tests/utils.py

@@ -243,6 +243,7 @@ class LightningNode(object):
         self.daemon = daemon
         self.bitcoin = btc
         self.executor = executor
+        self.known_fail = False
 
     # Use batch if you're doing more than one async.
     def connect(self, remote_node, capacity, async=False):
@@ -307,3 +308,9 @@ class LightningNode(object):
         c.close()
         db.close()
         return result
+
+    def has_failed(self):
+        """Note that a daemon has (deliberately) crashed, so we don't fail
+        on cleanup"""
+        self.known_fail = True
+