BigTable: Behaviour of BTType.mutationsToRow is different compared to actual BT.

[anish749]

The mutationsToRow defined in BigTableType, assumes col qualifiers to be utf8 strings (for sorting), and sorts the set cell requests before applying.

The above code creates as many columns as there are set cells. So when mutations contain the same col but different cell timestamps, this breaks. The expected behaviour would be for one col to have all the cells that belong to that column with different cell timestamps.

The row has the column qualifiers sorted, but the mutations aren't applied after sorting the col qualifiers. Mutations to the same row are applied serially (batch write is forced to serialize mutations to the same row).