db.c: Make db_vars_(rm|set) a no-op when nothing changed
db.c: Make db_vars_(rm|set) a no-op when nothing changed

Avoids some needless SQL queries. A hash table lookup is probably faster
than even passing a query to the background thread.

srijan/ncdc,srijan/ncdc,Tilka/ncdc,srijan/ncdc,Tilka/ncdc