Do is unresponsive while a verb is being executed

[wekyle]

Beacuse of the way the verb runner is written (that is, synchronously), Do is unable to handle any incoming requests while it's executing a verb sequence.

This may cause problems with Slack connectivity and state when running very long verb sequences.

We should refactor the verb runner to an asynchronous shell execution API, being careful to preserve the waterfall pattern of a sequence's execution.

[wekyle]

https://www.npmjs.com/package/promise-waterfall

This appears to solve this problem. Moving issue to 1.0.0 milestone!

[wekyle]

Ended up writing a custom promise 'unwrapper' to support creating an array of promises that execute in a waterfall fashion, while still returning the outputs like a map function would, in dc418f3.

Behind the scenes, we're using child_process.exec instead of child_process.spawnSync now.

Fixed!