DATAMONGO-852 - Update keeps track of fields to be modified.

Update holds a set of fields that modifications are registered for. This information is used to determine if a modification is registered for the `version` field of a versioned entity. 

The change was introduced since the present solution did not correctly find the version property within the `DBObject` resulting from the mapped update.

In case version property is already included in `Update` automatic version update via `$inc` will be skipped.