Remove synchronous database methods

This required doing additional caching at startup (e.g., item types and fields)
so that various methods can remain synchronous.

This lets us switch back to using the current Sqlite.jsm. Previously we were
bundling the Fx24 version, which avoided freezes with locking_mode=EXCLUSIVE
with both sync and async queries.

Known broken things:

  - Autocomplete
  - Database backup
  - UDFs (e.g., REGEXP function used in Zotero.DB.getNextName())