SCHEMA-82. Merge KijiCounter into KijiCell.

This is the bare minimum change to remove KijiCounters

notes
* If we include the entire location of the cell (family/qualifier/timestamp), there are also a bunch of methods like getRawCell(family, qualifier, timestamp), and maybe the 'address' of a cell needs to be its own class
* The KijiCell.equals() method seems to be only useful for tests (and hashCode is unsupported)
* AvroCellDecoder and CounterCellDecoder are still two separate classes
* I haven't changed LsTool