Keeping query context while changing query types

[fmassot]

Example to reproduce the lost of query context:

1. Enter a metrics query (term or date histogram)
2. Execute it
3. Change logs query
4. Go back to metrics query: you lost your previous query.

It would be nice to keep the metrics query context.

[ddelemeny]

This probably requires some degree of rewriting the state management in QueryEditor.

At the moment we use a stateless reducer that delegates the state management to grafana through mutations of the query. If we want to keep some sidecar state outside of the active query, we should introduce some local state management.