TLS: refactor Security::ContextPointer to a std::shared_ptr

TLS: refactor Security::ContextPointer to a std::shared_ptr

These pointers now use the same construction pattern tested out with
Security::SessionPointer.

It also fixes a reference counting bug in GnuTLS code paths where the
PeerConnector::initialize() method would be passed a temporary Pointer
and thus free the context/credentials before it was used by the session
verify logics.