1114 parallel requests

That was easy!
Check out the [diff without spacing](https://github.com/onaio/zebra/compare/1114_parallel_requests?w=1) for exactly what was changed. The trick is that `io/get-url` `io/raw-get-url` are non-blocking requests that write into channels. If we make sure to make all of these non-blocking requests before any of our blocking operations (which is take, or `<!`), our code is parallelized without too much effort!
@royrutto @geoffreymuchai Might be worth it to check out [this commit](https://github.com/onaio/zebra/commit/e39585be2743f060e271064ddcc9f9a2aaa512f3), which is the meat of this pull request.