Updating avatar URL and displayname at the same time results in weird m.room.member events (SYN-183)

This issue has been migrated from [#1251](https://github.com/matrix-org/synapse/issues/1251).

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Update both the avatar URL and displayname of a user who is in a few rooms at the same time. You will get **two** m.room.member events which have exactly the same content (with both keys changed). I would expect either:
- 2 m.room.member events with a single changed key per event.
- 1 m.room.member event with 2 changed keys.
But not 2 m.room.member events with 2 changed keys (the same ones).

This is annoying because it results in things like [SYWEB-145](https://matrix.org/jira/browse/SYWEB-145). It's also not easy to fix, because if you diff the `prev_content` and `content` you get the same diff (both avatar and name) twice, resulting in.. a duplicate message.

(Imported from https://matrix.org/jira/browse/SYN-183)

(Reported by @Kegsay)

#### Attachments:
https://matrix.org/jira/secure/attachment/10107/mroomember.png