Move from proxies to well-defined adapters

[fatkodima]

This is a work in progress (failing tests, missing new tests and documentation), but basically is ready for review to verify basic ideas.

To add new adapters, all is needed is inheriting from base class and implementing required methods:

module Rack
  class Attack
    class MyStoreAdapter < StoreAdapter
      ...

Closes #380

[fatkodima]

Updated this accordingly to master. Added tests. This pr became a little bold. @grzuy Please, review.

[grzuy]

This can be considered backwards incompatible change, so it should be considered for v7.x.

[fatkodima]

Hey, @grzuy Wdyt on moving this (and my other PRs) forward? Is it time for v7.x?