Refactor AbstractAppDataSnapshotServer so that subclasses don't need schemas

Currently AbstractAppDataSnapshotServer requires the users of the operator to set up the schemas, query deserializer, result serializer, query processor, etc. A simple query processing operator should not need all that. Let's have a more abstract app data server that only requires an embeddable query input operator, which is necessary if the query needs to be processed immediately without delay, which can be significant if the DAG has significant latency.