lightningd: always do incremental parsing of JSON inputs.

time lightning-cli -R --network=regtest --lightning-dir /tmp/ltests-k8jhvtty/test_pay_stress_1/lightning-1/ listpays > /dev/null

Before:
	real	0m42.741s
	user	0m0.149s
	sys	0m0.016s

After:
	real	0m13.674s
	user	0m0.131s
	sys	0m0.024s

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: JSON-RPC: significant speedups for plugins which create large JSON replies (e.g. listpays on large nodes).

lightningd/plugin.h

@@ -56,6 +56,8 @@ struct plugin {
 	/* Stuff we read */
 	char *buffer;
 	size_t used, len_read;
+	jsmn_parser parser;
+	jsmntok_t *toks;
 
 	/* Our json_streams. Since multiple streams could start
 	 * returning data at once, we always service these in order,