Implement TLS context cache

The intention of having a TLS context cache object is manyfold:

In the case of client-side contexts: allow reusing the previously created contexts to employ the context-specific TLS session resumption cache. That will enable XoT connection to be reestablished faster and with fewer resources by not going through the full TLS handshake procedure.
In the case of server-side contexts: reduce the number of contexts created on startup. That could reduce startup time in a case when there are many listen-on statements referring to a smaller amount of tls statements, especially when ephemeral certificates are involved.
The long-term goal is to provide in-memory storage for additional data associated with the certificates, like runtime representation (X509_STORE) of intermediate CA-certificates bundle for Strict TLS/Mutual TLS (ca-file).

Edited Dec 27, 2021 by Artem Boldariev