nostrdb/Query Plans

Instead of running queries off filters directly, we do some simple
heuristics and determine a reasonable query plan for the given filter.

To test this, also add a kind index query plan and add a test for it.

We still need tag, author, and created_at index scans. This is up next!

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/nostrdb.h

@@ -399,6 +399,11 @@ struct ndb_block_iterator {
 
 struct ndb_query_result {
 	struct ndb_note *note;
+	uint64_t note_id;
+};
+
+struct ndb_query_results {
+	struct cursor cur;
 };
 
 // CONFIG