proxy: rewrite stdlib ReverseProxy

The ReverseProxy code from the standard library doesn't actually
give us the control that we want. Pull it down and rip out what
we don't need, adding tests in the process.

All available endpoints are attempted when proxying a request. If a
proxied request fails, the upstream will be considered unavailable
for 5s and no more requests will be proxied to it. After the 5s is
up, the endpoint will be put back to rotation.
Merge pull request #1055 from bcwaldon/proxy-monster

proxy mode, pt II