Update getX509() callsites to OpenSSLTransportCertificate
Update getX509() callsites to OpenSSLTransportCertificate

Summary:
We need to migrate all callsites of `getX509()` to instances of
`OpenSSLTransportCertificate`. The concrete implementations of the existing
`AsyncTransportCertificate` should be folly's `BasicTransportCertificate` and
fizz's `SelfCert` and `PeerCert`s, each of which also implement the
`OpenSSLTransportCertificate` interface, so the dynamic cast should always
succeed for now.

Reviewed By: yfeldblum

Differential Revision: D27175178

fbshipit-source-id: 273b85dc98cd827795c62fdeb6e6fed49dd79806

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift