jsonrpc: reduce debug log spam for JSON commands.

Just log the failed ones, not every connection and successful commands.

Before (VALGRIND=0 -n10):
111 passed, 1 skipped in 175.78 seconds

After:
111 passed, 1 skipped in 173.92 seconds
111 passed, 1 skipped in 164.16 seconds
111 passed, 1 skipped in 171.30 seconds
111 passed, 1 skipped in 180.05 seconds
111 passed, 1 skipped in 180.04 seconds


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

lightningd/jsonrpc.h

@@ -19,6 +19,8 @@ struct command {
 	struct lightningd *ld;
 	/* The 'id' which we need to include in the response. */
 	const char *id;
+	/* What command we're running (for logging) */
+	const struct json_command *json_cmd;
 	/* The connection, or NULL if it closed. */
 	struct json_connection *jcon;
 	/* Have we been marked by command_still_pending?  For debugging... */