remove requirement to support AsyncApi

per issue #1501 AsyncApi will no longer be supported after jclouds 1.6. 

This issue makes it possible to implement apis without using async at all, providing the mechanisms work.

old | new | change
-----|----- |----
RestContext | ApiContext<A> | formerly misnamed, this suggests there's a single top-level api in the context.
RestApiMetadata |HttpApiMetadata<A> | suggests that the metadata related can generate http requests.  Couldn't use ApiMetadata, as that's the name of the interface.
RestClientModule | HttpApiModule<A> | corrects intent as many of our api are far from ReSTful

To prove the concept, this change includes a converted route53 provider, which succeeded in live tests.