[RFC] Make increased use of Invokable params cache

While this patch probably violates good design principles because of
the imperfect location of the cache, exposing the params cache outside
of RestAnnotationProcessor to S3Utils results in a roughly 40%
improvement in performance for generating signed GETs and PUTs for
S3. This commit also converts a few others calls to
Invokable.getParameters() but the observed benefit from those was
small in my microbenchmarks.

This is related to JCLOUDS 301.