Support `@JsonIdentityInfo` for Map-valued properties

Per https://github.com/FasterXML/jackson-databind/issues/1479#issuecomment-384053027

This issue is to request that `@JsonIdentityInfo` annotations work for Maps, when used to annotate a FIELD that is a `Map`

When serializing an annotated map, generate an ID for it. When deserializing a map with the annotation, first check for a reference to an existing Map.