The fix that disabled VTODO tests accidentally ran all other tests twice
for this storage.
This wasn't picked up earlier since Fastmail is not currently running on
CI.
Add asyncio to the storage backends and most of the codebase. A lot of
it merely uses asyncio APIs, but still doesn't actually run several
things concurrently internally. Further improvements will be added on
top of these changes
Thanks to Thomas Grainger (@graingert) for a few useful pointers
related to asyncio.
