make _measure_lags static

Change-Id: I8b2ce1370229c668858fa905dde794f399ca8412
initialize queue in __init__

Change-Id: If8d97ee11ce44753fc45e7874c57a4e4f99db1bc
unlock gil in multithreaded xdcr collector

Change-Id: Ifc3b33d2931fa17888454769efc3323f86a0a439