[refactor/redesign] decouple ObservableQuery's mutation and query logic

[micimize]

ObservableQuery is a bit a kitchen sink for reactive operation logic and complex mutation logic. This is far from ideal – graphql_flutter makes delicate use of the generic onData api exposed by graphql to execute callbacks, etc.

This could be substantially reworked. For example, a particularly ambitious contributor might leverage the gql_link system to decompose the various aspects of "observation" into different links.