Improve async message checks

[Leo-Besancon]

This improves checks on async messages.

• Avoid adding a message to the pool if the validity slot makes it not executable

• Better catch errors to avoid async message being partly executed (load module fail but transfer coin successful and not reverted)

• [ ] document all added functions

• [ ] try in sandbox /simulation/labnet

  • [ ] if part of node-launch, checked using the resync_check flag

• [ ] unit tests on the added/changed features

  • [ ] make tests compile
  • [ ] make tests pass

• [ ] add logs allowing easy debugging in case the changes caused problems

• [ ] if the API has changed, update the API specification

[Leo-Besancon]

I'm fine with the code. It would be nice to have a TU for each change. That would help to track the intent of the commit (but that's far from mandatory).

Thanks, I'll see what I can do :)