Add a DeserializerFeature that disables use of final fields for setting values

[smeder]

Discussion here: FasterXML/jackson-module-afterburner#42

It would be great if a new flag "ALLOW_FINAL_FIELDS_AS_SETTER" could be added. This flag would indicate if the code should try to set values on final fields. To preserve existing behavior this flag should default to true.

[cowtowncoder]

Lolz. Look at that: #190. So I added this feature already back in 2.2, as MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS.

Now, as to it being a MapperFeature: this is because all DeserializationFeatures and SerializationFeatures being available for change on per-call basis; whereas MapperFeature must be set once before using ObjectMapper. And since pruning of properties is something that can not be undone or changed, it belongs into this more fundamental set of settings, even if only applicable to deserialization.