DATAMONGO-1086 - Mapping fails for collection with two embbeded types that extend a generic abstract.

We now use the type information of the raw property type to check if we need to include _class.
DATAMONGO-1100 - Upgrade to new PersistentPropertyAccessor API.
DATAMONGO-941 - Add support for $min to Update.

Prepare issue branch.
DATAMONGO-941 - Add support for Update $min.

We now offer dedicated methods for ‘min’ (Number/Date) on Update.
As BigInteger and BigDecimal values are converted to String before storing in MongoDB it is not possible to use for the min operation. Therefore we type check both the given value and the property it should be applied to and raise an error if  it fails.
DATAMONGO-941 - Add support for Update $min.

Moved value field from KeywordContext into Keyword directly since it was always accessed directly. Made KeywordContext immutable. Validation now works with Keywords directly instead of using the KeywordContext.
Introduced static factory method keywordFor(...) for constructing keywords.