[db] Move calculation of artist/album id's from sqlite to code

The purpose of this is to support library backends making their own
calculation of these id's, which is relevant if they have more information
available than just album_artist and album.

This also removes a bunch of sqlite extension code plus some triggers, which
in itself is probably an improvement.

src/misc.h

@@ -130,6 +130,9 @@ swap_pointers(char **a, char **b);
 uint32_t
 djb_hash(const void *data, size_t len);
 
+int64_t
+two_str_hash(const char *a, const char *b);
+
 char *
 b64_decode(const char *b64);