* Add tests for StreamIdGenerator

* Drive-by: annotate all defs

* Revert "Revert "Remove slaved id tracker (#14376)" (#14463)"

This reverts commit d63814fd, which in
turn reverted 36097e88. This restores
the latter.

* Fix StreamIdGenerator not handling unpersisted IDs

Spotted by @erikjohnston.

Closes #14456.

* Changelog

Co-authored-by: Nick Mills-Barrett <nick@fizzadar.com>
Co-authored-by: Erik Johnston <erik@matrix.org>