End-to-end encryption: getKeysChanges

[chrisabruce]

Query users with recent device key updates.

Gets a list of users who have updated their device identity keys since a previous sync token.

The server should include in the results any users who:

• currently share a room with the calling user (ie, both users have membership state join); and
• added new device identity keys or removed an existing device with identity keys, between from and to.