Suggestion: Make middleware pipeline async with promises

[deslee]

Summary

To address some of the problems with hosting bolt as a serverless function (#220)

For example, in AWS Lambda, we must not call res.end() until the very end of our flow. This is due to https calls forcefully terminating after we ack with a response. The middleware pipeline needs to be async in order for us to know when bolt is finished with the event.

Another issue I haven't addressed in this PR is the fact that the receiver is using Node's EventEmitter class. The emit method does not return a promise or take a callback, so we won't be able to know when the serverless function can ack. My personal solution was to write a custom receiver using emittery, which provides an emitter that returns a promise. I feel like this is a bad practice however, and there is probably a better way to process an event in a way that's safe to run inside a serverless handler.

Using the changes in my forked repo, I was able to get a bolt-powered endpoint working in Zeit Now, which is powered by AWS Lambda.

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[codecov]

Codecov Report

Merging #238 into master will not change coverage.
The diff coverage is 52.17%.

@@          Coverage Diff           @@
##           master    #238   +/-   ##
======================================
  Coverage    62.5%   62.5%           
======================================
  Files           7       7           
  Lines         464     464           
  Branches      126     126           
======================================
  Hits          290     290           
  Misses        150     150           
  Partials       24      24

Impacted Files	Coverage Δ
src/ExpressReceiver.ts	51.42% <0%> (ø)	⬆️
src/conversation-store.ts	100% <100%> (ø)	⬆️
src/middleware/builtin.ts	39.82% <45.45%> (ø)	⬆️
src/App.ts	73.2% <50%> (ø)	⬆️
src/middleware/process.ts	56.25% <58.33%> (ø)	⬆️

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[seratch]

@deslee Thank you very much for taking the time to make this PR. Also, I'm very sorry for our belated response. 🙇

As you may be already aware, Bolt v2 has resolved the issue you mentioned here in a quite similar way. The latest version supports async functions for middleware/listeners.

So, please allow me to close this PR now. If you have feedback/suggestions on Bolt v2, we'd love to know them in a new GitHub issue.