Dcnum

- fix: negative argument for time.sleep

- enh: support boolean images in mask postprocessing

- ref: enable mask postprocessing by default for all segmenters
- ref: increment DCNUM_PPID_GENERATION to 9

- fix: slot_chunks were not performed in increasing order, making it
impossible for writer to continue writing and consequent OOM
- enh: reserve one CPU for writer thread and control logic
- enh: disentangle logging and debugging keyword arguments
- enh: do not initialize slot_chunks and slot_states with lock

- enh: improve strategy for stalling for slow writer

- ref: remove __init__ from SegmentThresh