This optimizes the Evaluator in the Authorize service to scale to very
large numbers of routes. Additional caching was also added when building
rego policy query evaluators in parallel to allow sharing work and to
avoid building evaluators for scripts with the same contents.
This also replaces instances where we manually write "return ctx.Err()"
with "return context.Cause(ctx)" which is functionally identical, but
will also correctly propagate cause errors if present.
