Add support to unix:// and tls:// connections to redis caching/sessions.

Issue created because @ MDL-66139 it was detected that our redis cache store does not support:

• tls:// connections (existing in older versions). Provides encrypted connections.
• unix:// connections (new since php-redis v5). Provides relative unix sockets.

(ref: https://github.com/phpredis/phpredis#connect-open)

So, this is about:

1) consider if it's worth supporting them.
2) implement them once agreed.

Also, worth noting that the same connection options should be working for redis sessions (haven't verified if they are already supported or no).

Ciao