logging: Removed automatic subprocess logging for bitcoind

This was responsible for a huge number of loglines simply because we
log every subprocess start and termination. Moving the logging
upstream to where it is really needed gets rid of the polling, that
are successful. A highly unscientific test shows a reduction in
loglines produced by lightningd from 17000 to 10000 lines.

daemon/bitcoind.c

@@ -131,7 +131,6 @@ static void bcli_finished(struct io_conn *conn, struct bitcoin_cli *bcli)
 	} else
 		*bcli->exitstatus = WEXITSTATUS(status);
 
-	log_debug(bitcoind->log, "reaped %u: %s", ret, bcli_args(bcli));
 	bitcoind->req_running = false;
 	bcli->process(bcli);
 
@@ -150,8 +149,6 @@ static void next_bcli(struct bitcoind *bitcoind)
 	if (!bcli)
 		return;
 
-	log_debug(bcli->bitcoind->log, "starting: %s", bcli_args(bcli));
-
 	bcli->pid = pipecmdarr(&bcli->fd, NULL, &bcli->fd, bcli->args);
 	if (bcli->pid < 0)
 		fatal("%s exec failed: %s", bcli->args[0], strerror(errno));
@@ -296,6 +293,7 @@ void bitcoind_sendrawtx_(struct peer *peer,
 				    int exitstatus, const char *msg, void *),
 			 void *arg)
 {
+	log_debug(bitcoind->log, "sendrawtransaction: %s", hextx);
 	start_bitcoin_cli(bitcoind, NULL, process_sendrawtx, true, cb, arg,
 			  "sendrawtransaction", hextx, NULL);
 }
@@ -310,8 +308,6 @@ static void process_chaintips(struct bitcoin_cli *bcli)
 		   struct sha256_double *tipid,
 		   void *arg) = bcli->cb;
 
-	log_debug(bcli->bitcoind->log, "Got getchaintips result");
-
 	tokens = json_parse_input(bcli->output, bcli->output_bytes, &valid);
 	if (!tokens)
 		fatal("%s: %s response",