Adds support for solid cache

[broosk1993]

This PR adds support for Solid Cache. It relies on the "unless_exist" flag to implement the locking mechanism. The flag was added recently: https://github.com/rails/solid_cache/pull/188/files.

The proper way to use this with Solid Cache is to create an initializer and put the following code in it:

require './lib/idempotent_request/policy'

Rails.application.config.middleware.use IdempotentRequest::Middleware,
                                        storage: IdempotentRequest::SolidCacheStorage.new(Rails.cache, expires_in: 1.day.from_now.to_f),
                                        policy: IdempotentRequest::Policy

The key difference here is using expires_in as opposed to expires_at.