allow rate limiting number of concurrent requests

[willnorris]

When loading a web page with a lot of proxy URLs so that the browser attempts to download them all immediately, the process is killed because it's out of memory. I don't know how to get around this, since it's not something I can control in browsers. It seems like imageproxy should be rate limiting these requests at some point to avoid OOM issues.

Originally posted by @aaronpk in https://github.com/willnorris/imageproxy/issues/97#issuecomment-546747249

[BrianGreenhill]

:+1: experiencing the same issue

[brian-themaven]

Any updates on this? We merged this code into our private fork and we're still seeing imageproxy being killed due to OOM.