Harmonize TTFB/TTLB definitions with Tor Metrics plots

Right now, OnionPerf plots 'command' to 'first_byte', whereas Tor Metrics plots 'start' to 'first_byte' (as found in legacy/trac#33258 (moved)). We should harmonize these definitions, which likely means using what we have been using for Tor Metrics plots for the past decade.

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