Critical Description
Currently we ignore most IgniteConfiguration properties if SpringConfigUrl is set.
So if there is even one property not propagated to .NET, user has to switch completely to Spring XML, which may be quite tragic, especially for query entity configurations.
Instead, we can merge configs on the top level:
First, we load Spring XML. Then we apply non-null .NET properties on top of it.
Primitive properties should be made nullable underneath to track whether user has set them.
