lightningd: make some bitcoind requests high priority.

fiatjaf has a cheap VPS, connecting remotely to his home bitcoind node.
    fiatjaf's latency on bitcoin-cli getblock is between 10 and 37 seconds.
    fiatjaf's c-lightning node is getting one block per hour.
    fiatjaf is sad.

We single-file our bitcoind requests, because bitcoind has a limited
thread pool and it *fails* rather than queueing if you upset it.  We
probably be fine using separate queues for each command type, but simply
allowing some requests to cut in line should prove my theory that we're
getting stuck behind gossip verification requests.

    fiatjaf now gets one block per 2 minutes.
    fiatjaf is less sad.

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

lightningd/bitcoind.c

@@ -247,6 +247,7 @@ start_bitcoin_cli(struct bitcoind *bitcoind,
 		  const tal_t *ctx,
 		  bool (*process)(struct bitcoin_cli *),
 		  bool nonzero_exit_ok,
+		  bool high_prio,
 		  void *cb, void *cb_arg,
 		  char *cmd, ...)
 {
@@ -274,7 +275,10 @@ start_bitcoin_cli(struct bitcoind *bitcoind,
 	bcli->args = gather_args(bitcoind, bcli, cmd, ap);
 	va_end(ap);
 
-	list_add_tail(&bitcoind->pending, &bcli->list);
+	if (high_prio)
+		list_add(&bitcoind->pending, &bcli->list);
+	else
+		list_add_tail(&bitcoind->pending, &bcli->list);
 	next_bcli(bitcoind);
 }
 
@@ -352,7 +356,8 @@ static void do_one_estimatefee(struct bitcoind *bitcoind,
 	char blockstr[STR_MAX_CHARS(u32)];
 
 	snprintf(blockstr, sizeof(blockstr), "%u", efee->blocks[efee->i]);
-	start_bitcoin_cli(bitcoind, NULL, process_estimatefee, false, NULL, efee,
+	start_bitcoin_cli(bitcoind, NULL, process_estimatefee, false, false,
+			  NULL, efee,
 			  "estimatesmartfee", blockstr, efee->estmode[efee->i],
 			  NULL);
 }
@@ -398,7 +403,8 @@ void bitcoind_sendrawtx_(struct bitcoind *bitcoind,
 			 void *arg)
 {
 	log_debug(bitcoind->log, "sendrawtransaction: %s", hextx);
-	start_bitcoin_cli(bitcoind, NULL, process_sendrawtx, true, cb, arg,
+	start_bitcoin_cli(bitcoind, NULL, process_sendrawtx, true, true,
+			  cb, arg,
 			  "sendrawtransaction", hextx, NULL);
 }
 
@@ -429,7 +435,8 @@ void bitcoind_getrawblock_(struct bitcoind *bitcoind,
 	char hex[hex_str_size(sizeof(*blockid))];
 
 	bitcoin_blkid_to_hex(blockid, hex, sizeof(hex));
-	start_bitcoin_cli(bitcoind, NULL, process_rawblock, false, cb, arg,
+	start_bitcoin_cli(bitcoind, NULL, process_rawblock, false, true,
+			  cb, arg,
 			  "getblock", hex, "false", NULL);
 }
 
@@ -457,7 +464,8 @@ void bitcoind_getblockcount_(struct bitcoind *bitcoind,
 					 void *arg),
 			      void *arg)
 {
-	start_bitcoin_cli(bitcoind, NULL, process_getblockcount, false, cb, arg,
+	start_bitcoin_cli(bitcoind, NULL, process_getblockcount, false, true,
+			  cb, arg,
 			  "getblockcount", NULL);
 }
 
@@ -619,7 +627,8 @@ static bool process_getblockhash_for_txout(struct bitcoin_cli *bcli)
 	/* Strip the newline at the end of the previous output */
 	blockhash = tal_strndup(NULL, bcli->output, bcli->output_bytes-1);
 
-	start_bitcoin_cli(bcli->bitcoind, NULL, process_getblock, false, cb, go,
+	start_bitcoin_cli(bcli->bitcoind, NULL, process_getblock, false, false,
+			  cb, go,
 			  "getblock", take(blockhash), NULL);
 	return true;
 }
@@ -640,7 +649,7 @@ void bitcoind_getoutput_(struct bitcoind *bitcoind,
 
 	/* We may not have topology ourselves that far back, so ask bitcoind */
 	start_bitcoin_cli(bitcoind, NULL, process_getblockhash_for_txout,
-			  true, cb, go,
+			  true, false, cb, go,
 			  "getblockhash", take(tal_fmt(NULL, "%u", blocknum)),
 			  NULL);
 
@@ -685,7 +694,8 @@ void bitcoind_getblockhash_(struct bitcoind *bitcoind,
 	char str[STR_MAX_CHARS(height)];
 	snprintf(str, sizeof(str), "%u", height);
 
-	start_bitcoin_cli(bitcoind, NULL, process_getblockhash, true, cb, arg,
+	start_bitcoin_cli(bitcoind, NULL, process_getblockhash, true, true,
+			  cb, arg,
 			  "getblockhash", str, NULL);
 }
 
@@ -697,7 +707,7 @@ void bitcoind_gettxout(struct bitcoind *bitcoind,
 		       void *arg)
 {
 	start_bitcoin_cli(bitcoind, NULL,
-			  process_gettxout, true, cb, arg,
+			  process_gettxout, true, false, cb, arg,
 			  "gettxout",
 			  take(type_to_string(NULL, struct bitcoin_txid, txid)),
 			  take(tal_fmt(NULL, "%u", outnum)),