Merge branch 'develop' into babolivier/message_retention

changelog.d/6273.doc

-Fix a small typo in `account_threepid_delegates` configuration option.
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changelog.d/6274.misc

-Port replication http server endpoints to async/await.

changelog.d/6275.misc

-Port room rest handlers to async/await.

changelog.d/6276.misc

-Add a CI job to test the `synapse_port_db` script.

changelog.d/6277.misc

-Remove redundant CLI parameters on CI's `flake8` step.
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changelog.d/6278.bugfix

-Fix exception when remote servers attempt to join a room that they're not allowed to join.

changelog.d/6279.misc

-Port `federation_server.py` to async/await.

changelog.d/6280.misc

-Port receipt and read markers to async/wait.

changelog.d/6284.bugfix

-Prevent errors from appearing on Synapse startup if `git` is not installed.
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changelog.d/6291.misc

-Change cache descriptors to always return deferreds.

changelog.d/6294.misc

-Split out state storage into separate data store.

changelog.d/6298.misc

-Refactor EventContext for clarity.
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changelog.d/6300.misc

-Move `persist_events` out from main data store.

changelog.d/6301.feature

-Implement label-based filtering on `/sync` and `/messages` ([MSC2326](https://github.com/matrix-org/matrix-doc/pull/2326)).

changelog.d/6304.misc

-Update the version of black used to 19.10b0.

changelog.d/6306.bugfix

-Appservice requests will no longer contain a double slash prefix when the appservice url provided ends in a slash.

changelog.d/6307.bugfix

-Fix `/purge_room` admin API.

changelog.d/6312.misc

-Document the use of `lint.sh` for code style enforcement & extend it to run on specified paths only.

changelog.d/6313.bugfix

-Fix the `hidden` field in the `devices` table for SQLite versions prior to 3.23.0.

changelog.d/6314.misc

-Replace every instance of `logger.warn` method with `logger.warning` as the former is deprecated.
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