`/keys/signatures/upload` is heavy and on the synapse master

This issue has been migrated from [#6843](https://github.com/matrix-org/synapse/issues/6843).

---

It took ~30s to 'store secrets' on my account, and similarly on Amandine's.

```
2020-01-24 22:15:56,841 - synapse.access.http.8080 - 233 - INFO - POST-8145074 - 65.127.237.10 - 8080 - Received request: POST /_matrix/client/unstable/keys/signatures/upload
2020-01-24 22:16:22,132 - synapse.handlers.device - 429 - DEBUG - POST-8145074 - Notifying about update '@matthew:matrix.org'/'RQRQBRNDOP', ID: 20726543
2020-01-24 22:16:22,134 - synapse.handlers.device - 438 - INFO - POST-8145074 - Sending device list update notif for '@matthew:matrix.org' to: { <4510 homeservers> }
2020-01-24 22:16:22,600 - synapse.access.http.8080 - 302 - INFO - POST-8145074 - 65.127.237.10 - 8080 - {@matthew:matrix.org} Processed request: 25.756sec/0.003sec (1.734sec, 0.415sec) (3.563sec/11.775sec/2210) 16B 200 "POST /_matrix/client/unstable/keys/signatures/upload HTTP/1.1" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.117 Safari/537.36" [0 dbevts]
```

Surely this shouldn't be on the master. (And should perhaps be an async API if it's going to take so long). See https://github.com/vector-im/riot-web/issues/12223 for the Riot side of this.