Improve redis memory usage check

[pelmered]

Improvements:

• Added ->warnWhenAboveMb() to set a warning threshold
• Sets memory usage in summary and meta data

[freekmurze]

Thanks!

[pelmered]

@freekmurze I'll probably add similar improvements to some other of the checks. For example DatabaseSizeCheck. Should I make one PR per check or just one big PR?

I'm also thinking about making checks for the following:

• Elasticsearch
• DB Snapshots (for your DB snapshot package)
• Sentry errors

Should I add them here or make my own third part packages?

[freekmurze]

I'd prefer separate PRs.

The new checks can go in packages of your own. Feel free to send a PR to our docs to add a link to them 👍

[pelmered]

Ok, great. I will do that!