* Message sending works again, but

 * Marking multiple devices known (ie. 'send anyway') is very slow
   because it writes all device info out each time
 * Support for non-indexedb stores not written yet
 * No migration

Don't end up with devices / device tracking status being null

Includes making saveIfDirty() return a promise in case you care
about when changes got saved (which the test does).

Allow localstorage store to take a localstorage impl, make TestClient
pass a cryptostore & fix True/true typo

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.

 * 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

 * Change test for new storage layer
 * Always store device keys we download, even if we weren't
   tracking the user.

It may change whilst processing the sync

This is more normal, and the code doesn't expect to get empty
objects here which is reasonable since it never sets one.