Initial 3.9.16 release notes

(cherry picked from commit 52ad930)
(cherry picked from commit 9682567)

Author: michaelklishin

release-notes/3.9.16.md

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+RabbitMQ `3.9.16` is a maintenance release in the `3.9.x` release series.
+
+Please refer to the **Upgrading to 3.9** section from [v3.9.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.9.0) if upgrading from a version prior to 3.9.0.
+
+This release requires at least Erlang 23.2, and supports Erlang 24. [RabbitMQ and Erlang/OTP Compatibility Matrix](https://www.rabbitmq.com/which-erlang.html) has more details on Erlang version requirements for RabbitMQ.
+
+
+## Changes Worth Mentioning
+
+Release notes are kept under [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.9.x/release-notes).
+Contributors are encouraged to update them together with their changes. This helps with release automation and a more consistent release schedule.
+
+### Core Server
+
+#### Enhancements
+
+ * Quorum queues: better forward compatibility with (currently in preview) RabbitMQ 3.10.
+
+   GitHub issue: [#4560](https://github.com/rabbitmq/rabbitmq-server/pull/4560)
+
+ * Significantly faster queue re-import from [definitions](https://rabbitmq.com/definitions.html)
+   on subsequent node restarts. Initial definition import still takes
+   the same amount of time as before.
+
+   GitHub issue: [#4538](https://github.com/rabbitmq/rabbitmq-server/pull/4538)
+
+ * Significantly faster exchange re-import from [definitions](https://rabbitmq.com/definitions.html)
+   on subsequent node restarts. Initial definition import still takes
+   the same amount of time as before.
+
+   GitHub issue: [#4585](https://github.com/rabbitmq/rabbitmq-server/pull/4585)
+
+ * RabbitMQ nodes will now filter out certain log messages related to
+   connections, channels, and queue leader replicas receiving internal protocol messages
+   sent to this node before a restart. These messages usually raise more questions
+   and cause confusion than help.
+
+   It is still possible to detect relevant underlying events (node or connection failures)
+   from other log messages.
+
+   GitHub issue: [#4557](https://github.com/rabbitmq/rabbitmq-server/issues/4557)
+
+#### Bug Fixes
+
+ * `rabbitmq-upgrade await_online_synchronized_mirror` is now a no-op in single node
+   clusters
+
+   GitHub issue: [#4570](https://github.com/rabbitmq/rabbitmq-server/pull/4570)
+
+
+## Dependency Upgrades
+
+ * `observer-cli` was upgraded [from v1.7.2 to v1.7.3](https://github.com/zhongwencool/observer_cli/compare/1.7.2...1.7.3)
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.9.16.tar.xz`
+instead of the source tarball produced by GitHub.