Use a single SSL context instead of 5 for TLS connections

[eakraly]

All versions of TLS can be supported using a single SSL_CTX - setting minimal/maximal supported version should be done using, for example, SSL_CTX_set_options(&turn_params.tls_ctx, SSL_OP_NO_TLSv1); (or using SSL_CTX_set_max_proto_version for openssl-1.1.0 and newer)

SSL_CTX_set_max_proto_version is not used (yet) and might introduce breakage if using openssl-1.1.1 and newer without API compatibility mode.

This is a first in series of changes to rework openssl usage.

Links: https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_options.html https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_max_proto_version.html