When the user has a certificate check callback set, we still have to
check whether the stream we're using is even capable of providing a
certificate.
In the case of an unencrypted certificate, do not ask for it from the
stream, and do not call the callback.
We currently have gitno for talking over TCP, but this needs to know
about both plaintext and OpenSSL connections and the code has gotten
somewhat messy with ifdefs determining which version of the function
should be called.
In order to clean this up and abstract away the details of sending over
the different types of streams, we can instead use an interface and
stack stream implementations.
We may not be able to use the stackability with all streams, but we
are definitely be able to use the abstraction which is currently spread
between different bits of gitno.
