Test compatibility with a YAML file saved by Mechanize::Cookie.
MozillaStore#close: Do not call SQLite3::Database#close if already closed.
Add tests for MozillaStore#upgrade_database and fix its bugs.
silently(): Use ensure.
Make MozillaStore#close actually "work" by closing open statements.

Add a finalizer to MozillaStore also, which automatically closes the
SQLite3 database.
Fix hand-made autoloading to allow directly referencing MozillaStore.
Inhibit cloning MozillaStore.
Comment out unused/advisory bits to gain coverage.