- UpdateHandle -> MergeHandle, InsertHandle -> CreateHandle
 - Also bunch of code cleanup in different places

 - Merged all filter* and get* methods
 - new constructor takes filestatus[]
 - All existing tests pass
 - FileGroup is all files that belong to a fileID within a partition
 - FileSlice is a generation of data and log files, starting at a base commit

  - Usage now marks code as clearly using either RO or RT views, for future evolution
  - Tests on all of FileGroups and FileSlices