Add new option, `JsonInclude.Include.NON_ABSENT`

[cowtowncoder]

(note: moved from https://github.com/FasterXML/jackson-datatype-jdk8/issues/8)

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Between existing options NON_NULL and NON_EMPTY, it would be good to have option that:

1. Excludes nulls (like both of above), but also
2. Excludes values of "referential types" that are considered empty (absent), that is, resolve to null
3. Do NOT exclude other "empty" values (like collections)

[cowtowncoder]

Support added; with core databind works for AtomicReference. Next need to add support for JDK8, Guava modules.