Introduce ConversionContext and clean up MappingMongoConverter

[mp911de]

To simplify customization of MappingMongoConverter, we should introduce a strategy object that encapsulates decision paths invoking simple and container conversion. Simple conversion maps to simple types. Container conversion applies conversion for lists, maps, and entities.

We also should cleanup the converter to align method signatures of the individual converter methods, reorder methods and revisit the converter surface.