Provide more clarity on when/how the cache store needs to be configured

[grzuy]

Try to avoid, as much as possible, any confusion from users about when and how they need to configure the cache store

• How it defaults depending on Rails presence
• When it is needed
• What type of cache store is suitable, needs to respond to which methods, etc

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• Give clearer error message in case using throttle but cache store is not set (Help users understand more clearly when the store is misconfigured #274)
• Give clearer error message in case using throttle but cache store is set but incompatible (Help users understand more clearly when the store is misconfigured #274)
• Give clearer error message in case using fail2ban but cache store is not set (Give clearer error message for misconfigured cache store for allow/fail2ban #315)
• Give clearer error message in case using fail2ban but cache store is set but incompatible (Give clearer error message for misconfigured cache store for allow/fail2ban #315)
• Give clearer error message in case using allow2ban but cache store is not set (Give clearer error message for misconfigured cache store for allow/fail2ban #315)
• Give clearer error message in case using allow2ban but cache store is set but incompatible (Give clearer error message for misconfigured cache store for allow/fail2ban #315)