Use memcmp as a certificate verifier

Since certificate verification can be quite slow and can be on the critical path for an app, it would be great to use a simple memcmp instead. To do this, we'd need the ability to provide a leaf certificate and the hostname it's valid to Envoy Mobile and then when verifying that the received certificate is valid, it could simple return true if the received cert is identical, otherwise fall back to the normal verification logic.