replace "DEL" Commands with "UNLINK".

[jonashrem]

Implements #158

This PR replaces DEL Command with UNLINK command like I suggested in #158

Details about UNLINK can be found here https://redis.io/commands/unlink .

As noticed here https://github.com/redis/redis/issues/1748#issuecomment-43233084 this will not change the behavior.

As there is no check of Redis version included, it will break Redis Versions < 4.0 through.

As Redis 4.0 was released on 2017-07-14 and Redis 3 went EOL with the Redis 5 on 2018-10-17, I don't expect this being an issue.

[nemphys]

@colinmollenhour not anything fancy, I just add a temporary Mage::log(sha1($script)); right after the script assignment line and copy it from the log.

[colinmollenhour]

Finally merged! Sorry it took over 2 years...

[jonashrem]

Thanks. I think we can close #158 than as well