Reduce cachability of Map deserializers, to avoid problems with per-property config changes

[cowtowncoder]

Looks like there are some problems in extended cachability of Map deserializers, related to polymorphic value handling. While it is not exactly clear what triggers this, it seems prudent to prevent caching in cases where values are polymorphic types. In future it may be possible to either fix the cause for conflicts, or reduce need for caching. But for now, let's try to resolve immediate issue seen with JAXB annotations module.

[cowtowncoder]

Note: this is a minor regression due to #604