Maintain DB Integrity During Local Scan

[tkem]

Currently, unreferenced albums and artists are deleted in SQLiteLibrary.close(), so these may still show up after an SQLiteLibrary.flush(). Therefore, cleanup code should be moved to triggers.

[tkem]

It may also make sense to wrap library actions which involve multiple SQL queries (e.g. the new browsing implementation with seperate queries for albums and tracks) in explicit transactions.

[tkem]

May be more of an issue now that it is recommended to set scan_flush_threshold to a lower value.