Un-Buffered Kryo Serialization

Currently, cascading.kryo writes to a byte buffer in memory which is then written out to the output format.  When serializing large objects this is a waste of memory and will contribute to OOM errors.  This patch introduces an option whereby cascading.kryo instead writes directly to the output stream (note that since the SequenceFile.writer itself has the same sort of buffer, this is only half of the solution to the above problem).

This comes at the cost of CPU, since it is still crucial to know the size of the serialized object in bytes prior to writing (both for compatibility with the  current "buffered" serialization, and because on deserialization, the size must be known -- to prevent kryo from reading beyond object boundaries and screwing hadoop).  Thus I invoke kryo serialization twice, once to write to a fake output stream which just counts how many bytes would have been written, and then again to the hadoop-owned output stream for actual writing.  This way the size of the serialized object, and its serialization are both computed without the necessity of a memory buffer large enough to hold the thing.

The flag is called "cascading.kryo.unbufferd" and defaults to false.  Note that since the bytes written to disk are the same irrespective of this flag, it is possible to read and write sequence files generated under a different value of this flag.