C#: Fix performance issue in EntityFramework modelling

[hvitved]

This PR fixes a perfomance issue in how we model flow through EntityFramework SaveChanges methods. When data reaches a SaveChanges method call, we jump (using SyntheticGlobals) to the relevant DbSet<elementType> properties, and we were previously storing unnecessarily much information in the SyntheticGlobals, which could result in a combinatorial explosion.