refactoring re-used code in CassandraServer classes into a common superclass.

This is my first attempt at refactoring common code between avro and thrift CassandraServer classes into a common superclass.

(It's not fully tested yet, but I did create and query a simple keyspace with cassandra-cli. Looking for feedback about the direction at this point rather than commit to repository).

Here is the broad outline:

1. A new class called AbstractCassandraServer that implements get() and insert() for now.
2. Specialized CassandraServer...Exception() classes thrown by StorageProxy, instead of re-using thrift exceptions.

As I progress in this refactoring, I'd like to do the following :

3. move validation fully into a shared class rather than avro and thrift doing validation.
4. Change batch_mutate so that it does not have avro/thrift versions.

As well as any other work along the way that will make the client-facing interfaces independent of the RPC protocol.

thanks!