DATAMONGO-1005 - Improve cycle-detection for DbRef's.

Introduced ObjectPath that collects the target objects while converting a DBObject to a domain object. If we detect that a potentially nested DBRef points to an object that is already under construction we simply return a reference to that object in order to avoid StackOverFlowErrors.

Original pull request: #209.
DATAMONGO-1005 - Polishing introduction of ObjectPath.

Simplified implementation of ObjectPath to use a static root instance and hand the path further down until final resolution in MappingMongoConverter.readValue(…). This removes a bit of boxing and unboxing code both in ObjectPath and the converter.

Introduced ObjectPath.getPathItem(…) to internalize the iteration to find a potentially already resolved object.

Renamed parameters and fields of type ObjectPath to path consistently. Removed obsolete method in MappingMongoConverter.

Original pull request: #209.