Support single @Id field which may be any unique sortable value (e.g. ObjectId)

[nealeu]

Currently our AbstractMappingFuzzyRepository implementation expects @id to be a String representation of the internal fuzzydb Ref.

Ref should only be used if explicitly specified, and the preferred external support should be @id based on UUID or BSON ObjectId (which is more compact).

The mapping layer should be refactored to allow any @id field to be used, to create an index on that field (using DataOperations.ensureIndex() ), and for findOne() to look up from that index to get the internal reference for the object with which to then do update, delete operations.