apps: fix shared request state

proper fix for #424
Right.

I don't have strong feelings, but my gut told me it would be more digestible if all of the metrics for `nsq_to_http` were based on the same unit of work.  What you're proposing means that, when in `ModeAll`, the aggregate metrics do not mean the same thing as the per-address metrics.  Are you OK with that single edge case?