Add support for "merge" annotation, to use existing POJO/Map/Collection property value

[cowtowncoder]

By default, Jackson will try to construct a new property value Object, regardless of its type, and for POJO/Map/Collection types add contents into that new Object. The main alternative would be to first check if property already has a value, and if so, use that value instead.
Currently such "merge" approach can only be used for 2 cases:

1. For root value, via "updateValue()" for ObjectReader
2. For Map/Collection values, if (and only if!) there is no setter method (or visible field)

It seems, however, that the ability to specify "merge" as the default approach (but fall back to new Object in case property is null) would be useful. This could be supported via annotation; either a new one, or perhaps additional property to an existing one (@JsonProperty(valueCreation=CREATE/MERGE/MERGE_ONLY))... or ideally something better).

If this can be made to work on per-property basis, we could perhaps also consider ability to specify global default (alternative to "always create").

[cowtowncoder]

Seems like I have created dups... since #1399 is closer to my current values, keeping that as open, closing this as dup.