Tracking request receipt timing on the test server per worker

[oschaaf]

Leaving a note here inspired by the discussion over at https://github.com/envoyproxy/nighthawk/pull/477#discussion_r476803233

Tracking the delta's between inbound requests per worker at the test server may yield interesting data. In particular, it might be useful in exposing unbalancedness in the way traffic gets spread across workers. This in turn may be useful to quantify effects of different pooling strategies across connection pools from intermediaries and clients.