Keep an up-to-date v1 GetState object in the master's memory.

In MESOS-10026, the operator API read performance was improved by directly serializing out unversioned protobuf held in memory to serialized v1 protobuf. This required a lot of manual writing out of serialized protobuf or json that we could eliminate if we had v1 state held in memory.

Holding v1 state in memory could be tackled by "self-susbcribing" (although probably without going through http) to the master's event stream and updating an overall GetState object based on each event.

Note that MESOS-8163 suggests adding a "state actor", such an actor would be implemented using the approach outlined in this ticket.