[Bug/Enhancement] Redis: "Memory limit" issue and Redis "monitoring"

[exodus4d]

There is a significant problem with installations running "Redis" either as "Cache backend" and/or as "Session handler".

Problem:

If Redis is configured with maxmemory-policy allkeys-lru, (recommended see: wiki Cache configuration), Redis will drop cache keys/data if maxmemory gets reached.
This behavior is nice for temporary cached data e.g. parsed templates, Ajax responses,... but could lead to problems when Session data gets auto-deleted for an online user (assuming you use Redis as Session handler too) and/or when data gets auto-deleted supposed to be cached infinite.

Info:

• There is no "warning/error" send from Redis if "used memory" exceeds the maxmemory limit.
• Redis has a INFO command that gives us feedback about the count of evicted keys due to memory limitations. Use this to quick-check if your Redis server needs more RAM assigned:

  $ redis-cli INFO STATS
  

How to fix:

• If evicted keys > 0 → increase maxmemory.
  
• If used_memory_peak_human > or close to maxmemory_human → increase maxmemory.
  

New Redis monitoring panel on /setup page:

I added a new "info panel" with current Redis stats (only visible if Redis is used as Cache backend):

• 64MB is the new requirement for Redis installations
• current used_memory and used_memory_peak data shown
• maxmemory-policy requirement → "allkeys-lru"
• evicted_keys count shown → try to keep this at "0"
• DBSIZE counts all cached keys