removed locked notify_all() call.

[GevArakelyan]

I'm not an expert but as far as i know when conditional variable notify_all() function is called it's discouraged to lock the mutex at the same time.

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[AppVeyorBot]

❌ Build benchmark 1388 failed (commit 8226d5c726 by @GevArakelyan)

[coveralls]

Coverage decreased (-0.008%) to 86.869% when pulling 1857756 on GevArakelyan:master into af441fc on google:master.