feerate: use u32 everywhere.

The wire protocol uses this, in the assumption that we'll never see feerates
in excess of 4294967 satoshi per kiloweight.

So let's use that consistently internally as well.

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

channeld/commit_tx.h

@@ -20,7 +20,7 @@ struct sha256_double;
  * received HTLCs.
  */
 size_t commit_tx_num_untrimmed(const struct htlc **htlcs,
-			       u64 feerate_per_kw, u64 dust_limit_satoshis,
+			       u32 feerate_per_kw, u64 dust_limit_satoshis,
 			       enum side side);
 
 /**
@@ -49,7 +49,7 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 			     enum side funder,
 			     u16 to_self_delay,
 			     const struct keyset *keyset,
-			     u64 feerate_per_kw,
+			     u32 feerate_per_kw,
 			     u64 dust_limit_satoshis,
 			     u64 self_pay_msat,
 			     u64 other_pay_msat,