Change SQL/PGQ properties in client data to extension client data

[Dtenwolde]

unordered_map<string, shared_ptr<ClientContextState>> registered_state; 
//! ClientContextState is virtual base class for ClientContext-local (or Query-Local, using QueryEnd callback) state
//! e.g. caches that need to live as long as a ClientContext or Query.
class ClientContextState {
public:
    virtual ~ClientContextState() {};
    virtual void QueryEnd() = 0;
};

subclassen en onder een alias opslaan in context:

context->registered_state.push_back("sqlpgq", pgq_state) |

[Dtenwolde]

Closed as of #40