(note: follow-up to #4499)

So: looks like #4082 has caused many problems and although direction is clear wrt canonical (de)serialization of Optionals (basically what https://github.com/FasterXML/jackson-modules-java8/ adds), it seems reasonably to add a MapperFeature that basically disables checks #4082 adds.

What this means is that when the new MapperFeature (Name To Be Decided) is changed:

1. java.util.Optional handling proceeds the way it would with no checks -- specifically, it is fine if default Bean-style handling occurs
2. No exception is thrown for missing explicit java.util.Optional handlers

It may seem odd to add a "feature to disable another newish feature", but this seems minimal path that achieves goals of #4082 while still allowing backwards-compatibility for users that depended on old handling (esp. with explicit visibility changes).