Bigtable: add helper methods to Rows to efficiently fetch qualifier values

[igorbernstein2]

A Row is currently a flat list of cells. The cells have a particular order: all cells for a column family are clustered (NOTE: column families are not sorted, only clustered.) and then sorted lexicographically by qualifier. To expose this behavior to the user we should have 3 methods that on the Row class:

List<RowCell> getCells(String family)
List<RowCell> getCells(String family, ByteString qualifier)
List<RowCell> getCells(String family, String qualifier)

These methods should efficiently return a list of matching cells. To implement this efficiently, the Row class should include Map of index ranges for each family cluster, which should populated by the RowMerger. Then the 3 methods could use a combination of index lookups (for the family) and binary searches (for the qualifier) to return the matching cells.