btcjson+rpcclient: restore bitcoind compatibility

The PR btcsuite#1594 introduced a change that made the order of parameters
relevant, if one of them is nil. This makes it harder to be backward
compatible with the same JSON message if an existing parameter in
bitcoind was re-purposed to have a different meaning.

btcjson/chainsvrcmds.go

@@ -822,7 +822,14 @@ func NewSearchRawTransactionsCmd(address string, verbose, skip, count *int, vinE
 // SendRawTransactionCmd defines the sendrawtransaction JSON-RPC command.
 type SendRawTransactionCmd struct {
 	HexTx         string
+	MaxFeeRate    *int32
 	AllowHighFees *bool `jsonrpcdefault:"false"`
+}
+
+// SendRawTransactionBitcoindCmd defines the sendrawtransaction JSON-RPC command
+// that is used for bitcoind post 0.19.0.
+type SendRawTransactionBitcoindCmd struct {
+	HexTx         string
 	MaxFeeRate    *int32
 }
 
@@ -842,8 +849,8 @@ func NewSendRawTransactionCmd(hexTx string, allowHighFees *bool) *SendRawTransac
 // sendrawtransaction JSON-RPC command to a bitcoind node.
 //
 // A 0 maxFeeRate indicates that a maximum fee rate won't be enforced.
-func NewBitcoindSendRawTransactionCmd(hexTx string, maxFeeRate int32) *SendRawTransactionCmd {
-	return &SendRawTransactionCmd{
+func NewBitcoindSendRawTransactionCmd(hexTx string, maxFeeRate int32) *SendRawTransactionBitcoindCmd {
+	return &SendRawTransactionBitcoindCmd{
 		HexTx:      hexTx,
 		MaxFeeRate: &maxFeeRate,
 	}

rpcclient/rawtransactions.go

@@ -357,7 +357,7 @@ func (c *Client) SendRawTransactionAsync(tx *wire.MsgTx, allowHighFees bool) Fut
 		return newFutureError(err)
 	}
 
-	var cmd *btcjson.SendRawTransactionCmd
+	var cmd interface{}
 	switch version {
 	// Starting from bitcoind v0.19.0, the MaxFeeRate field should be used.
 	case BitcoindPost19: