Block X-Backend* in gatekeeper

By moving the blocking to gatekeeper from the proxy server, we gain
the ability to pass X-Backend headers in via InternalClient while
still keeping real clients from using them.

I wanted this functionality while working on storage policies; I had
an InternalClient and wanted to tell it to use a specific policy index
instead of what the container said, and that seemed like a good time
for an X-Backend header.

Change-Id: I4089e980d3cfca660365c7df799723b1f16ba277
Merge "Block X-Backend* in gatekeeper"