The Context in the http.Request is bound to the one we give it when
calling balancedBackend.do(), so we were accessing it out of scope
when calling the final json.Decode on the response body.

This was not a problem for most *small* requests, as the response Body
already contains all the data due to having read it along with the
headers. However, larger response bodies would cause json.Decode to
call a Read with what at that point is a canceled Context, so the Call
function would return a mysterious "context canceled" error.

(Note that this change introduces a minor, probably less annoying,
issue, where we can't reset "resp" between successive calls so we may
be invoking json.Decode on a non-pristine object if the first call
fails in some circumstances).

Allows for failover in case a target times out.

To match the previous calling convention for DoJSONHTTPRequest and
reduce confusion when migrating from the old API.

Implement a simpler API for the Backend interface, removing most old
public methods and replacing them with a single Call() method, making
the package look a bit more like an actual rpc package (so, hopefully,
easier to replace in the future).