Requests persistence optimization by using cache (optional)

Optionally activate cache persistence setting REQUEST_USE_CACHE to True.
Cache prefix can be configured with the REQUEST_CACHE_PREFIX setting.

Every time a query is made against Request database table, the cached requests will be stored in the database.
If you want to manually persist your cache to the database with a command or a cron task, make sure to use the Request.objects.persist_cached function.

Refactored some Request class methods into its object manager.