Switch to using httputil's ReverseProxy

Go provides a standard solution for reverse proxies, which could be used in place of the current backend request proxy code:

https://golang.org/pkg/net/http/httputil/#ReverseProxy

The issue does not name a file or test; start by locating the current backend request proxy code and compare its behavior with Go's net/http/httputil ReverseProxy documentation. Done means the existing proxy behavior is preserved while the standard ReverseProxy implementation is used, with relevant tests still passing.

Written by the indexing model from the issue text.