Support for parallel evaluation of required authorization features for a given permission

[ihinsdale]

Parallel, as opposed to serial, evaluation would yield better performance, but it also potentially introduces non-determinism in the case of multiple failing authorization features, which would make debugging more difficult. (Unless, in the case of failure, we wait for all features to evaluate and return one of them deterministically.) Probably best to make this feature optional for those who are willing to accept the tradeoff.