Perf-up the AuthZ!

[Kahbazi]

• Reuse the AuthenticateResult, AuthorizationResult, AuthorizationFailure, DenyAnonymousAuthorizationRequirement instances
• Store Task<Policy> to avoid creating a new Task on each call.
• Use foreach instead of Any in authz requirements to avoid the closure allocations.

[ghost]

Thanks for your PR, @Kahbazi. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[davidfowl]

Very nice!

[Tratcher]

Thanks