Faster remote room joins: add an error code to /members when in partial state

[matrixbot]

This issue has been migrated from #14587.

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For now the request is just stalled until the join completes, which is not ideal since clients are not aware of what is happening.

This requires a MSC to define the error code.
For backward compatibility we also probably need to add a parameter to /members so that the client can opt-in to the specific error code.
Or perhaps returning a 5xx is enough ? https://stackoverflow.com/questions/9794696/which-http-status-code-means-not-ready-yet-try-again-later has several suggestions on that subject.