Change MapperFeature.USE_GETTERS_AS_SETTERS) default to false

[cowtowncoder]

This setting can be highly confusing at first, and it seems better to require explicit "opt-in" here.
Originally feature was added after default behavior had been set as equivalent of true, which is where default setting comes from (which in turn is because JAXB has this behavior).
But with 3.x we change defaults to be more ergonomic, less confusing, so let's do this.

[cowtowncoder]

Fixed: FasterXML/jackson-databind#1916