C# client SDK: implement multiplicity tracking

[gefjon]

Alongside each row in the client cache, store a refcount. Fire insert/delete events only on the edge between 0 and 1.

Definition of Done: it is possible to subscribe to two queries which have at least one row contained in both of them. That shared row is shown to client cache queries as if it was present once, as in, the client cache remains set semantic. After unsubscribing from one query, the row remains present because it is still referenced by the other query.

[joshua-spacetime]

Closed by #249