Provide a faster / cleaner path for "mandatory" Messages

[gbin]

We changed the API to allow the tasks to send Optional Payloads in messages.
This is important for example when you have merging tasks that might not have a 1 to 1 matching messages from 2 different sources.

But it added the burden and a little overhead for all task to have to check the Option<> for all payloads even if by construct they might not need to be optional.

Idea: make another type of messages that are like the old ones (with no Option<>) and provide an option in the macros and the DAG to use them.