OpLabelVolume optimization

[burgerdev]

• a volume with singleton z axis (aka 'image') is now given to vigra.analysis.labelImageWithBackground (improves speed)
• locking is done on thread level instead of on request level (removed potential deadlock with overlapping requests)
• the operator is using really abstract classes
• writing an implementation for lazy CC labeling should be easier now

[burgerdev]

Should be safe, tested both lazyflow and ilastik - with and without blockedarray. Waiting for thorbenk/blockedarray#4