test/test_protocol: add fee support.

We simply record how many fee changes there are, rather than supporting
a particular level.

Fees are tricky: it's a noop to apply them when incoming, but we apply them
when they've been acked.  Unlike HTLC modifications, which are symmetric,
fee updates only apply when returning to the originating node.

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

test/commits/13-fee.script.expected

@@ -0,0 +1,24 @@
+***A***
+OUR COMMITS:
+ Commit 1:
+  Offered htlcs:
+  Received htlcs:
+  Fee level 1
+  SIGNED
+THEIR COMMITS:
+ Commit 1:
+  Offered htlcs:
+  Received htlcs:
+  SIGNED
+***B***
+OUR COMMITS:
+ Commit 1:
+  Offered htlcs:
+  Received htlcs:
+  SIGNED
+THEIR COMMITS:
+ Commit 1:
+  Offered htlcs:
+  Received htlcs:
+  Fee level 1
+  SIGNED