Support for multi-valued headers

[odusseys]

AWS added support for multiple-valued headers see here

We will be forking as this is urgent for us, but will probably submit a PR if our solution works.

[piotrkubisa]

If you like forks then I've prepared one for you https://github.com/piotrkubisa/apigo 😛

I've forked the apex/gateway, implemented multi-value query strings, headers, introduced new event-to-http.Request conversion algorithm that is easier to customize, also done some profiling and squashed some allocations and nanoseconds needed for L7 proxy, as you can see in following benchmark (https://gist.github.com/piotrkubisa/03e04aaa8a4c5992189c556596f12689).

[tj]

not too worried about the performance side of things, if you use API Gateway for that kind of traffic it's going to be like 1000x the cost of running on EC2 :D, happy to merge a PR for the multi-header stuff if anyone needs it