Test and document failure and recovery scenarios

[jruaux]

Document how Smart Cache behaves in the following scenarios:

• Redis becomes unavailable
• Source of record becomes unavailable
• Redis database is dropped (i.e., database is flushed)\

What happens when Redis fails? Node failure, Cluster failure, etc.?
What if the end user has reduced database infrastructure because of caching?
When Redis comes back online, what is the behavior?
What's the behavior when the source of record is unavailable?
What's the behavior when Redis is unavailable?