Synapse should track `Clock.sleep(...)` to cancel during `shutdown`

[MadLittleMods]

Currently, we only track Clock.call_later(...) calls but Clock.sleep(...) can equally hold references to the homeserver and need to be cleaned up during shutdown in order for the homeserver to be garbage collected.

Spawning from https://github.com/element-hq/synapse-small-hosts/pull/204#discussion_r2525000337

Follow-up to #18828