Benchmarks and Additional Integration Tests

[aajtodd]

Add more robust benchmarks and integration tests. I'm not convinced of the implementation I've settled on for how a PolicyManager stores policies and relates them to a principal (via explicit attach/detach). I'd like to explore other designs before settling on one but to do that we need a more robust benchmark suite to compare implementation strategies. We should probably enumerate possible strategies and their perceived tradeoffs before embarking on this.

We also need better integration tests which would allow us to use the same test suite across different backend manager implementations.