Stale device lists when users re-join e2e rooms

[richvdh]

Yet another stale device list bug:

• Alice and Bob share an e2e room; Bob tracks Alice's devices
• Bob leaves the room, so stops getting updates
• Alice adds a new device
• Alice and Bob start sharing a room again
• Bob has an out of date list of Alice's devices

[richvdh]

Proposed fix is to apply two fixes to both the /sync and /keys/changes APIs:

• Add a new field left. When Bob and Alice stop sharing any rooms, Bob's server puts Alice's user id in this field, so that the client knows it will no longer receive updates:

  GET /sync
  
  200 OK
  {
      "device_lists": {
          "changed": [ … ],
          "left": ["@alice:matrix.org", … ],
      },
      …
  }
  

• When Alice rejoins a room that Bob is in (or vice versa), Bob's server notes that (a) Alice they now share a room and (b) they did not share a room before, and adds Alice's user to the 'changed' list.

[richvdh]

This has been fixed for riot-web in matrix-org/matrix-js-sdk#535 and matrix-org/synapse#2443; note that it will require development versions of both synapse and riot, currently.

Fixes for Android (element-hq/riot-android#1603) and iOS (matrix-org/matrix-ios-sdk#340) remain outstanding.