Apparently the dummy transaction hasn't been necessary since 2007 or so

https://bugzilla.mozilla.org/show_bug.cgi?id=380345
Remove use of deprecated pragma default_cache_size

Instead, set cache_size at runtime
Asynchronous DB query methods (experimental)

See comment in db.js for example usage.

This requires Firefox 20 or later unless we bundle the necessary code
modules ourselves.