Parallel threads on mailboxes

[heindel]

The main unused potential of multiple mailboxes¹ is a canonical division of (part of) engine state such that we can spawn several threads in parallel on several mailboxes during an isolated turn of an engine process. On the level of specs, we may want to have tasks as building blocks of actions.

In summary, I propose to have

• parallel threads, each one working on a mailbox, only changing the mailbox state and queue, producing send and spawn request as usual, but not updating the environment of the "encompassing" engine
• each action execution may involve several tasks

In particular, the current choice to combine several actions into a single one is flawed and should be fixed along the way.

This issue comes with a quick draft of code changes:
#303

Footnotes

1. Right now, little use of mailbox identifiers is made. That may have several reasons. One may be that having a natural number alone is not such a great design choice. ↩