lightningd: suppress feerate debug messages when nothing changes.

Every 30 seconds, these flood the logs.

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

lightningd/chaintopology.c

@@ -288,6 +288,7 @@ static void update_feerates(struct bitcoind *bitcoind,
 	bool changed = false;
 
 	for (size_t i = 0; i < NUM_FEERATES; i++) {
+		if (satoshi_per_kw[i] != topo->feerate[i])
 			log_debug(topo->log, "%s feerate %u (was %u)",
 				  feerate_name(i),
 				  satoshi_per_kw[i], topo->feerate[i]);