Add yet another SerializationFeature, WRITE_ABSENT_MAP_VALUES

[cowtowncoder]

With addition of new JsonInclude.Include option, NON_ABSENT, and existing dualism between POJOs, java.util.Maps, seems like we will also need WRITE_ABSENT_MAP_VALUES.
Its semantics are such that if enabled, it will exclude serialization of both Java nulls AND "referential nulls" (like Optional.absent() and new AtomicReference<>(null)); if disabled, WRITE_NULL_MAP_VALUES is consulted.

Yes, things get bit hairy, and it would be cleaner if instead we could set JsonInclude.Include value instead. But it has to do.

[cowtowncoder]

After trying to implement this, I realized one challenge: while it is relatively straight-forward to work for Map-valued properties, as they typically have proper value type declaration, it will not work quite right for root-Maps where type is generally assumed to be Object.
Given that users are probably going to be surprised by one case working, others not, I will defer implementation for now.

[cowtowncoder]

Quick note: with 2.7, there is the new concept of content inclusion (as distinct from older value inclusion) -- that is, related but separated checks for container, its contents -- this may be easier to support. Or possibly even works as is; if latter, needs to be tested to verify that specific combination of inclusion definitions works as expected.

[cowtowncoder]

Actually, I think that other new features (like config overrides for JsonInclude) should take care of this.