Make JAXB-based providers endpoint-scoped

JAXB providers have static Maps of JAXB contexts, additionally default JAXB providers are shared between all the endpoints which prevents the JAXBContexts from being recycled.

Dynamically cloning the default providers seems to be the best option at this stage; adding them early to endpoint-specific ProviderFactory instances is problematic as they can interfere with other custom XML/JSON providers in cases where they are deemed to be equal by the sort algorithm or when custom providers have weaker media types (with wildcards). Adding them dynamically might create some minor contention but only at the start.