Support passing client certificate chain in TLS handshake with Redis server

[rturner3]

It is currently not possible to benchmark a cluster that has TLS enabled where the memtier's client certificate is signed by an intermediate CA. The tool has a config flag --cert, but it only reads in a single certificate, not a chain of certificates including the intermediate CAs required for the Redis server to verify the client certificate back to a trusted CA certificate.

It appears that this might be fixable by exchanging the usage of the OpenSSL function SSL_CTX_use_certificate_file() from here with SSL_CTX_use_certificate_chain_file(), see OpenSSL documentation: https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_use_certificate_file.html