Implement Sticky Events MSC4354 (#5028)
* Implement Sticky Events MSC
* Renames
* lint
* some review work
* Update for support for 4-ples
* fix lint
* pull through method
* Fix the mistake
* More tests to appease SC
* Cleaner code
* Review cleanup
* Refactors based on review.
* lint
* Store sticky event expiry TS at insertion time.
* proper type
