Use mem_cache_store instead of dalli_store

`dalli_store` is deprecated since dalli 2.7.11.

We can now enable cache_versioning. We didn't enable it when upgrading
to Rails 5.2 because of possible incompatibility with `dalli_store` [1],
even though apparently some the issues were fixed in dalli 2.7.9 and
dalli 2.7.10 [2].

Since using cache versioning makes cache expiration more efficient, and
I'm not sure whether the options we were passing to the dalli store are
valid with memcache store (documentation here is a bit lacking), I'm
just removing the option we used to double the default cache size on
production.

[1] https://www.schneems.com/2018/10/17/cache-invalidation-complexity-rails-52-and-dalli-cache-store
[2] https://github.com/petergoldstein/dalli/blob/master/History.md

config/application.rb

@@ -28,10 +28,6 @@ class Application < Rails::Application
     # Use local forms with `form_with`, so it works like `form_for`
     config.action_view.form_with_generates_remote_forms = false
 
-    # Keep disabling cache versioning until we verify it's compatible
-    # with `:dalli_store` and with the way we cache stats
-    config.active_record.cache_versioning = false
-
     # Keep using AES-256-CBC for message encryption in case it's used
     # in any CONSUL installations
     config.active_support.use_authenticated_message_encryption = false

config/environments/preproduction.rb

@@ -57,7 +57,7 @@
   # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)
 
   # Use a different cache store in production.
-  config.cache_store = :dalli_store, { value_max_bytes: 2000000 }
+  config.cache_store = :mem_cache_store, { value_max_bytes: 2000000 }
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
   # config.action_controller.asset_host = "http://assets.example.com"

config/environments/production.rb

@@ -56,7 +56,7 @@
   config.log_tags = [:request_id]
 
   # Use a different cache store in production.
-  config.cache_store = :dalli_store, { value_max_bytes: 2000000 }
+  config.cache_store = :mem_cache_store
 
   # Use a real queuing backend for Active Job (and separate queues per environment)
   # config.active_job.queue_adapter     = :resque

config/environments/staging.rb

@@ -57,7 +57,7 @@
   # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)
 
   # Use a different cache store in production.
-  config.cache_store = :dalli_store
+  config.cache_store = :mem_cache_store
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
   # config.action_controller.asset_host = "http://assets.example.com"