Make indexedDB to default fast cache by default

Turns out it's currently the fastest among the three currently
implemented (Cache, browser.storage.session, indexedDB). Possibly
because indexedDB can natively persist structure-cloneable data,
something uBO can now benefit with the work on abstracting away
the limitations of various storages being limited to persist only
text or JSON data.

Related issue:
uBlockOrigin/uBlock-issues#2969