WiP: first test to try to resolve #507

[FCO]

If red-do :transaction's block returns a Awaitable, it creates
a new Promise that will handle the commit/rollback and does not
let the sync code to call commit/rollback.

Probable next steps are:

• extract that logic to a transaction manager
• on transaction manager add a transaction stack and methods
  • queue-begin
  • unqueue-commit
  • break-queue-rollback
    (or something like that)