Update diesel to 2.*

[data-sync-user]

We were holding off on updating to diesel 2.2 due to some complications, when outside contributor tommie provided this update as part of https://github.com/mozilla-services/syncstorage-rs/pull/1644 . Since there was a cargo audit alert for diesel, updating is suggested, we can cherry pick this update out of #1644, which should help reduce the size of tommie’s final patch.

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