Change implementation of "delay" to a single asyncio task

[AlanCoding]

As I started on an implementation of a "timeout" feature in #52, this was put in a single asyncio task. This is probably slightly better than creating a new task for each individual thing that needs a timeout. However, the "delay" feature, implemented in the main.py module right now creates a new task for every delayed task. This somewhat complicates error handling, and you have to cancel an arbitrary list of tasks when shutting down. It would also be better to have a constant number of asyncio tasks globally, so that they would be countable generally.

So this issue asks for a change in the implementation that the main logic puts delayed tasks in a single task. Purely refactoring.