[OpLabelVolume] reduce code duplication

[burgerdev]

I read some lines of code in OpCompressedCache recently, and realized that I had rewritten some multithreading and block handling logic in OpLabelVolume. Major changes are:

• OpLabelVolume.Output operates on a WYRIWYG basis (what you request ...) with no cache involved
• blocked labeling was disabled because
  1. the PR Connected Components python interface thorbenk/blockedarray#4 is not merged yet
  2. the blocked labeling operator has to be rewritten to satisfy new requirements by OpLabelingABC
  3. chances are that we have a lazy labeling operator in the near future

ilastik is not affected, because

• OpLabelVolume is used only in thresholding
• thresholding is using only OpLabelVolume.CachedOutput
• OpLabelVolume.CachedOutput's API did not change

All lazyflow tests (except some dvid test) are passing on my machine.