Asynchronous Uploads

[sumnerevans]

Very WIP Asynchronous Uploads support

Implements the recently merged MSC2246 for Synapse's media repository.

Closes #12484

• Configuration options rate limiting media creation
• Configuration options for how long to wait until media IDs expire
• POST /_matrix/media/v1/create
• PUT /_matrix/media/v3/upload/{serverName}/{mediaId}
• timeout_ms support on /download
• timeout_ms support on /thumbnail
• Option to restrict how many open media creations a given user can have.

Signed-off-by: Sumner Evans sumner@beeper.com

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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• Pull request includes a sign off
• Code style is correct
  (run the linters)

[sumnerevans]

Closing in favor of #15503