Modify DbSet to not use CommandGenerator directly

[LukasSefcik]

• Modify DbSet to not use CommandGenerator directly, but only IQueryProvider.
• CommandGenerator is class for creating commands for DbSet. But in case of database connection, it relies on IQueryProvider. IQueryProvider creates instance of DbCommand, because it knows database connection. Because of that, there is useless method GetCommandForCurrentTransaction.
• It would be great if the DbSet will be dependent only on IQueryProvider and IQueryProvider will use CommandGenerator inside. Consider deleting CommandGenerator and moving code to IQueryProvider.