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).

See !5672 (merged), #1784 (closed)