Patch `rake cache:clear` for large amounts of keys

lib/tasks/cache.rake

 namespace :cache do
   desc "GITLAB | Clear redis cache"
   task :clear => :environment do
-    Rails.cache.clear
+    # Hack into Rails.cache until https://github.com/redis-store/redis-store/pull/225
+    # is accepted (I hope) and we can update the redis-store gem.
+    redis_store = Rails.cache.instance_variable_get(:@data)
+    redis_store.keys.each_slice(1000) do |key_slice|
+      redis_store.del(*key_slice)
+    end
   end
 end