Move from proxies to well-defined adapters

[grzuy]

An attempt to address #316

What?

• Create a cache store Adapter interface, that defines which are the minimal set of methods that rack-attack needs to function properly.
• Transform existing proxies into adapters, which implement the defined Adapter interface. E.g. "redis adapter", "memcache adapter".

Why?

End users benefits

• More clear on what and what not to expect when reading/accessing Rack::Attack.cache.store
• Less confusion when configuring rack-attack cache store backend
• Less friction to introduce new cache store backends
  • Well-defined interface eases development and testing of new adapters
  • plus is easier for anyone to code and release new adapter as separate gem if they want, instead of being forced to get approval upstream

Contributors & Maintainers benefits

• Easier to test
• Easier to debug cache store issues
• Easier to implement support for new cache store backends
• Better maintainability in general

[grzuy]

Less friction to introduce new cache store backends

This benefit has been already addressed by #453