One-time-key upload/claim is racy

* Alice tries to upload one-time-key, but doesn't get response
* Bob claims the key
* Alice uploads the key again
* Charlie claims the key

Only one of Bob or Charlie can use the key to talk to Alice.

No file or test is named in the issue. Begin by locating the Matrix specification sections covering one-time-key upload and claim, then define the expected behavior for the interrupted upload and retry scenario; done requires the specification to make clear that a key cannot be used by both Bob and Charlie.

Written by the indexing model from the issue text.