cache: replace memcache with redis.

Drop memcache. Use redis for all caching instead.

config/initializers/cache_store.rb

@@ -0,0 +1,26 @@
+Rails.application.configure do
+  if Rails.env.test?
+    cache_config = [:memory_store, { size: 32.megabytes }]
+  else
+    cache_config = [
+      :redis_cache_store,
+      {
+        url: Danbooru.config.redis_url,
+        namespace: nil,
+        connect_timeout:   30, # default: 20 seconds
+        write_timeout:    0.2, # default: 1 second
+        read_timeout:     0.2, # default: 1 second
+        reconnect_attempts: 0, # default: 0
+        error_handler: ->(method:, returning:, exception:) {
+          DanbooruLogger.log(exception, method: method, returning: returning)
+        }
+      }
+    ]
+  end
+
+  cache_store = ActiveSupport::Cache.lookup_store(cache_config)
+
+  config.cache_store = cache_store
+  config.action_controller.cache_store = cache_store
+  Rails.cache = cache_store
+end