Add support for outputing sstables

This is a fairly major rewrite. The idea is that the mapper will output
Columns rather than rows. This way we can sort them correctly into the
reducer so that we can handle RangeTombstones.

This also gives us several other benefits. One is that we can just send
the Cassandra Atoms across to the reducer, which keeps us from having to
process Json while in flight, avoiding encoding/decoding problems that
we had previously on characters that didn't serialize into json without
escaping.

Because of this the final output is now an SSTable. It just makes the
whole process of incremental processing easier because we only support
one fileformat rather than SSTables and Json.

To get back to Json we will add a new serialization format when we
deprecate the old way of processing files.