Expose the current key to the CombineFn

One can not currently access the key during combining. Having access to the key can be useful during various phases.

There is a less then ideal workaround where the user embeds the key within the value turning the combiner over <K, V> into a combiner over <K, <K, V>>.

Originally asked in https://stackoverflow.com/questions/56451796/how-can-i-access-the-key-in-subclass-of-combinerfn-when-combining-a-pcollection