Fix IndexedDB xTruncate cache interaction

[rhashimoto]

The current implementation of xTruncate for IndexedDB pre-dates the addition of caching. In addition to updating the size in metadata, it deletes IndexedDB block objects past the EOF. If dirty blocks past EOF are in the cache, they will be written again on xSync.

The proper fix here is probably:

• In xTruncate:
  • Update size in metadata (as before).
  • Remove blocks past EOF from the cache, not from IndexedDB.
• In xSync:
  • After flushing the cache, delete IndexedDB block objects past EOF.