Cancellation of timeouts

[jmalloc]

This issue a place for discussion about how to (conceptually) cancel a pending timeout message. This is, by design, currently not an explicit feature. Instead, a timeout is canceled implicitly when the process that created it is ended.

The reasoning behind this decision is nuanced. The main influencing factors are:

• The asynchronous/non-deterministic nature of message-based applications
• The desire to encode domain logic using business language, as opposed to technical language, as much as possible
• A desire to avoid "double handling" of timeout cancellation logic

I think we should have a verbal discussion about this, from which I will take some notes to include here and in an ADR. We can use our somewhat typical example of an invoicing system that using a timeout message to mark invoices as overdue.

/cc @koden-km