Summary:
This is a start to wrapping various SSL objects going forward so different
binaries can choose different version of OpenSSL (i.e., BoringSSL, OpenSSL
1.1.0, OpenSSL 1.0.2, etc.). There's no change to the caller - everyone just
uses 'SSLSession', but the implementation details vary

Reviewed By: siyengar

Differential Revision: D3707791

fbshipit-source-id: f895334a768cb7d43b41af40c9bc06be5307cc7f