Media repository reads files from disk on the main thread.

This issue has been migrated from [#9312](https://github.com/matrix-org/synapse/issues/9312).

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It does use [`FileSender`](https://twistedmatrix.com/documents/current/api/twisted.protocols.basic.FileSender.html), which reads the file in chunks and so shouldn't be *too* slow, but nonetheless there is potential for disk I/O to take a long time (especially with networked storage).

We should look into either a) doing async file I/O (though that is likely hard) or b) do the reads from a thread.