Merge tag 'v1.18.0rc2' into develop

Synapse 1.18.0rc2 (2020-07-28)
==============================

Bugfixes
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- Fix an `AssertionError` exception introduced in v1.18.0rc1. ([\#7876](https://github.com/matrix-org/synapse/issues/7876))
- Fix experimental support for moving typing off master when worker is restarted, which is broken in v1.18.0rc1. ([\#7967](https://github.com/matrix-org/synapse/issues/7967))

Internal Changes
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- Further optimise queueing of inbound replication commands. ([\#7876](https://github.com/matrix-org/synapse/issues/7876))