Implement concurrent requests when crawling

[leonstafford]

Can revisit this after previously failed attempts with curl_multi on old codebase. Should be much cleaner now with Guzzle and async promises.

Still curious as to concurrent request limits on certain hosts, but if there's a user option to scale it down, at least, should be fine

[leonstafford]

thought: performance-wise, would fetching static files first speed things up on a 10,000+ post site? ie, not taxing the server as much/prone to creeping memory usage vs dynamic pages....