modules/stats: further improve latency measurements

modules/stats: further improve latency measurements

libuv's updating the stamp just once per event loop
might be too coarse, as processing multiple packets
(e.g. DNSSEC validation) may take milliseconds together.

Of course we still don't measure e.g. the time when incoming
client requests stay in OS buffers.

(cherry picked from commit 5e6a02b3902ac98b327eca281ae70fa2bb9a9598)