Add KeyspaceChange CqlResultType

High level clients want to handle failover and load balancing transparently to the application, which means not just connection pooling but moving an existing connection to another server if necessary. When this happens, the client needs to know what the active keyspace was before failover, so it can set it to the same one in the new connection.

Currently some clients handle this by checking for "SET KEYSPACE" queries, which violates the design principle that clients shouldn't have to parse CQL. Adding a new CqlResultType (that is set in response to a SET KEYSPACE command) would make this unnecessary.