Look into support for secure TLS connections to memcached/redis servers

Resources: https://github.com/memcached/memcached/blob/master/testapp.c https://wiki.openssl.org/index.php/SSL/TLS_Client https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_new.html

Related to https://github.com/twitter/twemproxy/issues/583

This would probably be done by

• Adding a tls: true section to twemproxy pool configuration files
• Adding a ./configure option to depend on openssl
• Changing callers of nc_write/nc_writev to call a helper method for the nc_conn instead