Speed up cleaning by reducing inter-thread interaction
Speed up cleaning by reducing inter-thread interaction

Replace atomic updating of shared statistics with private thread statistics
that are merged after cleaning completes.

mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex