feat: allow async stream handlers

[achingbrain]

Allow awaiting promises inside stream handlers without needing to do Promise.resolve().then(async () { ... }).

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[achingbrain]

Refs: ipshipyard/roadmaps#22