Improve on timers in io_uring_context

[maikel]

The current implementation submits an operation for each timer that is active. This makes it impossible to have a large number of timers at once with io_uring_context.

This issue is a reminder that timers should be handled differently. For example, we could build an intrusive heap of timers and only submit the root timer. Or we copy how libunifex is doing it.