Stop Jackson from using getters as setters.

This commit is stripped down from airlift/airlift@a7aa030c9d6d6d91c5f55977d481ca0bb9dd8c4b. I don't accept that Jackson should be disabled from accessing private fields/methods, the bug in Jackson that Dain is addressing notwithstanding.

be544688e8a68a9a1ffb1339a85a3827e97ccca7 demonstrates the bug. I think we need to take a practice that Jackson serializers and deserializers should be separate classes when they are asymmetric.