No database-level where clauses were being used in many cases they should be.
Some .osu file sources (or database sources) may be in a bad state. We want to get these ones right as they cannot be enforced by the database itself (via FKs or constraints).
Caused a stall as experienced in #1364.
Main fix is avoiding nullrefs being thrown when metadata isn't present on a beatmap (quite a common scenario).
Because we are not sharing a single context, we need to use Update to attach the entity to the local context.