* Check whether we share an e2e room with user IDs in the 'left'
field of /keys/changes: there's no guarantee we no longer share
any e2e rooms with these users
* Reset everyone's tracking status on an initial sync - just
re-fetching device lists for all users we're currently tracking
isn't good enough since room memberships may have changed.
* Fix typo in test
Mostly making tests aware of new storage format or making them
force it to be written. Also some bugfixes like we didn't json
encode some things in the localstorage store and we didn't
correctly check the promise when requesting device data saves.
When we no longer share any rooms with a given user, the server will stop
sending us updates on their device list, and will (once synapse is updated)
send us a notification of that fact via the 'left' field in the device_lists
field in /sync, or the response from /keys/changes.
There's a fuzzy line between the megolm tests and the devicelist ones, but
since I want to add more tests for devicelists, we might as well put the ones
which are definitely about devicelists in their own file
