HttpCache Purge Functionality

[tedivm]

Symfony's HttpCache has an issue open trying to deal with automated invalidation. Essentially, the HttpCache is filling up but can't be removed in an automated console based way because the HttpKernel is unavailable.

By creating a Store wrapper around the Pool object we can then be free to continue reusing the Pool in other contexts, such as the console. Then when we create the "Purge" console command we can have it cover this as well.