Promise.all for vector of requests

[sirmspencer]

We can optionally pass in a vector of requests instead of just one. As far as I can tell they are are independently launched and the on success is fired for each when it finished. Is there any pattern we can use to mimic a promise.all? That is the multiple calls have a single on success / on failure event that is called after all of the calls resolve.

[mike-thompson-day8]

Some discussion here: day8/re-frame#808

(Not for the promise.all case.)