Add WP_REDIS_SSL setting, pass to predis and phpredis

[david-donachie]

Simple fix to allow a WP_REDIS_SSL setting to pass SSL connection arguments in the case of a TLS service (such as Heroku) that needs additional SSL parameters.

connect_using_phpredis() sets an 'ssl' key on it's connection arguments derived from it via $parameters['ssl'], connect_using_predis() directly uses the $parameters['ssl'] value.

e.g.

    define('WP_REDIS_SSL', [
        'verify_host' => false,
        'verify_peer' => false,
        'verify_peer_name' => false,
    ]);

[tillkruss]

Closes #433.

[david-donachie]

I believe I've done all the requested changes

• WP_REDIS_SSL changed to WP_REDIS_TLS
• WP_REDIS_TLS added to README.md and to diagnostics.php
• WP_REDIS_TLS parsed to $parameters['ssl'] for predis
• WP_REDIS_TLS parsed to $args['others']['stream'] for phpredis > 5.3.0
• Tested with both predis and phpredis

[david-donachie]

@tillkruss I think I've made all the changes you requested, but I still see the PR marked as requesting further changes. Is there something I've not yet done? Our deadline is approaching quickly, so if you need me to do something else, just let me know.