MAHOUT-1615: drmFromHDFS returning the same Key for all Key,Vec Pairs for Text-Keyed SequenceFiles

SparkContext.sequenceFile(...) will yield the same key per partition for Text-Keyed Sequence files if the key a new copy of the key is not created when mapping to an RDD.  This patch checks for Text Keys and creates a copy of each Key if necessary.    
.. and since we know that Writables are not be useable since they are reused, we probably should block this case completely out with an error?

There's another piece of information to consider. Spark itself definex implicit conversions from some well-known Writables to their payload types. Perhaps we should support everything that's there; and maybe even figure a way to automatically apply everything that Spark exports, without even doing cases. I tried to figure how to do that (i remember that) but still haven't figured. it may not be possible; at least, i remember i haven't figured how that might be done. But we are at least 1.5 years past that moment, so perhaps we could revisist this from a fresh perspective. It would require eyeballing Spark's implicit Writable conversions again.