[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/db_init.h

@@ -26,7 +26,7 @@
  * is a major upgrade. In other words minor version upgrades permit downgrading
  * forked-daapd after the database was upgraded. */
 #define SCHEMA_VERSION_MAJOR 20
-#define SCHEMA_VERSION_MINOR 01
+#define SCHEMA_VERSION_MINOR 02
 
 int
 db_init_indices(sqlite3 *hdl);