[OpLabelVolume] reduce code duplication

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 thorbenk/blockedarray#4 is not merged yet
    1. the blocked labeling operator has to be rewritten to satisfy new requirements by `OpLabelingABC`
    1. 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.