performance: heartbeats and possibly timers are more expensive than required

[crusso]

A hearbeat is always executed in a separate message, causing 2 extra messages (3 in total). We can probably reduce the hearbeat to just a single message using an async* variant of the system method, or just 2 messages by using the fire-and-forget -1 callbacks when making the call.

A similar optimization might apply to timers, though we want to be careful to isolate the timer actions from each other and the maintenance of the priority queue (depends on how this is implemented).

https://dfinity.slack.com/archives/CPL67E7MX/p1732189506932589