``redis`` basic installation

[Mischback]

rspamd relies on redis as a fast caching backend.

This issue is directly related to #15.

Requirements

• [ ] install redis from Debian/stable
• [ ] provide a basic configuration
  • [ ] only available locally (bind to localhost)
  • [ ] limit RAM usage (needs investigation)
  • [ ] write to disk frequently (needs investigation)
• [ ] Can redis be used for other required caching?
  • [ ] Postfix?
  • [ ] nginx
  • [ ] Django