Remove unused queryId after fetchMore

[doomsower]

This PR addresses issue #2286 (and #3901)

Every call of fetchMore on ObservableQuery causes another query to be created my queryManager. After this query gets executed, its result is merged into parent query's result. But child query is not disposed, it just sticks around in queryManager.queries/ queryManager.queryStore and does nothing. As far as I understand, this is a leak, and this query should be disposed.

Please correct me if this is desired behavior, or if this change can produced any unexpected side-effects.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[benjamn]

Thanks for sowing that friendly doom, @doomsower!