Setting profile + joining a room can race (SYN-787)

This issue has been migrated from [#1444](https://github.com/matrix-org/synapse/issues/1444).

---

The Slack bridge appears to be triggering a race condition of some kind in synapse that most regular users would be too slow to provoke. Namely, that if concurrently a client tries to set a displayname/avatar image and join a room, the member event in the room never ends up with that profile information.

For example, the user `@​_slack_decentralizedweb_U1BLXG620:matrix.org` is in the room `!WBhcGXTDMlzyTPWoJv:matrix.org` (which is `##decentralizedweb-general:matrix.org`. The user themself does have a profile set:

```
$ matrixtool client -u @​LeoNerd:matrix.org json /_matrix/client/r0/profile/@​_slack_decentralizedweb_U1BLXG620:matrix.org
{
"displayname" : "pfrazee",
"avatar_url" : "mxc://matrix.org/SBrnbsCSCpVFLZOArSPNaSni"
}
```

However, their `m.room.member` event in the room does not contain this information:

```
$ matrixtool client -u @​LeoNerd:matrix.org json /_matrix/client/r0/rooms/'!WBhcGXTDMlzyTPWoJv:matrix.org'/state/m.room.member/@​_slack_decentralizedweb_U1BLXG620:matrix.org
{
"membership" : "join",
"displayname" : null,
"avatar_url" : null
}
```

(Imported from https://matrix.org/jira/browse/SYN-787)

(Reported by @leonerd)