Extend Descriptor to include a format value and refactor reader/writer apis

As part of a wider effort to improve the performance of our storage engine we will need to support basic pluggability of the SSTable reader/writer. We primarily need this to support the current SSTable format and new SSTable format in the same version. This will also let us encapsulate the changes in a single layer vs forcing the whole engine to change at once.

We previously discussed how to accomplish this in CASSANDRA-3067