(stable) Don't use "phutil_hashes_are_identical()" to compare public keys

Summary:
Ref T13436. There's no real security value to doing this comparison, it just wards off evil "security researchers" who get upset if you ever compare two strings with a non-constant-time algorithm.

In practice, SSH public keys are pretty long, pretty public, and have pretty similar lengths. This leads to a relatively large amount of work to do constant-time comparisons on them (we frequently can't abort early after identifying differing string length).

Test Plan: Ran bin/ssh-auth --sshd-key ... on secure with ~1K keys, saw runtime drop by ~50% (~400ms to ~200ms) with ===.

Maniphest Tasks: T13436

Differential Revision: https://secure.phabricator.com/D20875