Improve RowDataSnapshot usage - round 2

[lemonboston]

Since the rowDataSnapshot.data(column, mapFunction) usage doesn't lend itself to used in lazy declarations, we can reconsider the interface and support classes again.

The proposal is to have String as return type and use Single and Optional adapters:

interface RowDataSnapshot<Contract> extends Iterable<String>
{
    Optional<String> stringData(String column);

    Optional<byte[]> byteData(String column);
}


Single<Color> color = new ColumnData<>(rowDataSnapshot, Tasks.COLOR, colorMapFunction);

Optional<Color> color = new OptionalColumnData<>(rowDataSnapshot, Tasks.COLOR, colorMapFunction);

For the common types we can create specific classes or use the general ones:

Single<Long> timestamp = new ColumnData<>(rowDataSnapshot, Tasks.TIME, Long::valueof);
Optional<Long> timestamp = new OptionalColumnData<>(rowDataSnapshot, Tasks.TIME, Long::valueof)

Single<Long> timestamp = new LongColumnData<>(rowDataSnapshot, Tasks.TIME);
Optional<Long> timestamp = new OptionalLongColumnData<>(rowDataSnapshot, Tasks.TIME);