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

Currently, we only track [`Clock.call_later(...)`](https://github.com/element-hq/synapse/blob/edc0de9fa00210975dad2d3faa27639ea9b7f5e2/synapse/util/clock.py#L252-L341) calls but [`Clock.sleep(...)`](https://github.com/element-hq/synapse/blob/edc0de9fa00210975dad2d3faa27639ea9b7f5e2/synapse/util/clock.py#L87-L95) can equally hold references to the homeserver and need to be cleaned up during [`shutdown`](https://github.com/element-hq/synapse/blob/edc0de9fa00210975dad2d3faa27639ea9b7f5e2/synapse/server.py#L420-L517) 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 https://github.com/element-hq/synapse/pull/18828