Clarify lifetime of TLS stack dependencies

[reneme]

Currently, TLS::Client and TLS::Server take a number of dependency objects by reference in their constructors. The application is responsible for keeping those objects alive as long as the TLS stack is alive.
Some or all of those dependencies might be shared with multiple instances of the TLS stack.

This is a follow-up to a discussion in the Session_Manager restructuring PR.

The "dependency objects" in question are:

• TLS::Callbacks
• TLS::Session_Manager
• TLS::Policy
• Credentials_Manager
• RandomNumberGenerator

To me, the most obvious change would be to replace the references by std::shared_ptr<>.