This adds a new `tls-client-cert-file` and `tls-client-key-file`
configuration directives which make it possible to use different
certificates for the TLS-server and TLS-client functions of Redis.
This is an optional directive. If it is not specified the `tls-cert-file`
and `tls-key-file` directives are used for TLS client functions as well.
Also, `utils/gen-test-certs.sh` now creates additional server-only and client-only certs and will skip intensive operations if target files already exist.
* Introduce a connection abstraction layer for all socket operations and
integrate it across the code base.
* Provide an optional TLS connections implementation based on OpenSSL.
* Pull a newer version of hiredis with TLS support.
* Tests, redis-cli updates for TLS support.
