Do not set startWork_ in EventBase if time measurement is disabled
Do not set startWork_ in EventBase if time measurement is disabled

Summary: startWork_ is used only if enableTimeMeasurement is set. It gives ~0.5% performance win for mcrouter.

Reviewed By: yfeldblum

Differential Revision: D2590176

fb-gh-sync-id: 07f2189ebdec751cd0d91d191d8f595780d2808a

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