Pass the ?pool query param through to the backend.
This will slice off the ?pool= param and pass it to the WebSocket
request, which requests that the Zoo API use a particular pool of
engines. This isn't something any users of the zoo api require; but it's
needed for the internal engine Zoo development workflow. This may be
used in the future, but for now this'll be always enabled. Passing any
value in the production servers will result in a "no backend" error for
now.
