Restore ForwardFeeMonitor's ability to see forwarding fees

Fixes ([#222])

Prior to ElementsProject/lightning@780f32d (`v23.05`) both `fee` and
`fee_msat` were sent for compatibility.  The ForwardFeeMonitor was
checking for the presence of the `fee` field before processing the
record.  This needed to be updated to `fee_msat`.

Boss/Mod/ForwardFeeMonitor.cpp

@@ -29,7 +29,7 @@ void ForwardFeeMonitor::start() {
 		try {
 			auto payload = n.params["forward_event"];
 			if ( !payload.has("out_channel")
-			  || !payload.has("fee")
+			  || !payload.has("fee_msat")
 			  || !payload.has("resolved_time")
 			  || !payload.has("received_time")
 			   )

tests/boss/test_forwardfeemonitor.cpp

@@ -131,7 +131,6 @@ int main() {
     "in_msat": "100001001msat",
     "out_msatoshi": 100000000,
     "out_msat": "100000000msat",
-    "fee": 1001,
     "fee_msat": "1001msat",
     "status": "settled",
     "received_time": 1560696342.368,