Implement CassandraKeyspace and CassandraTable.

Also Connection.useKeySpace() and CassandraClient.getKeyspace() and createKeyspace().
Make sure that Connection automatically reconnects when the connection was lost.

This is required for ConnectionPool to work correctly and also convenient for the non-vibe.d version. Also made a bunch of methods in Connection private or protected to avoid misuse.
Add Table.exists, Keyspace.name and Keyspace.client properties.