Removes overly tight coupling between skip list store classes

o Moves serialization from SkipListStoreRecord to StoreView
o Factors out SkipListStoreCabinet thanks to that
o Adds SkipListStoreLoadedRecord for the cases where we actually need to
  keep the dynamic records around for reuse
o Renames LabelStore to LabelScanStore
o Deletes remnants of old implementation
Sketches skip list key intersection