PREF: optimise preloading application

We preload to ensure as much memory as possible is reused from unicorn master
to various workers using copy-on-write (sidekiq, unicorn)

This migrates the preloading code into the Discourse module for easier
reuse and adds 3 notable preloading changes

1. We attempt to localize a string on each site, ensuring we warmup
the i18n

2. We preload all our templates (compiling .erb to class)

3. We warm-up our search tokenizer which uses cppjieba which is a large
memory consumer, this will only cause a warmup on CJK sites or sites with
the special site setting enabled.

config/unicorn.conf.rb

@@ -53,43 +53,15 @@ initialized = false
 before_fork do |server, worker|
 
   unless initialized
-    # load up the yaml for the localization bits, in master process
-    I18n.t(:posts)
-
-    # load up all models and schema
-    (ActiveRecord::Base.connection.tables - %w[schema_migrations versions]).each do |table|
-      table.classify.constantize.first rescue nil
-    end
-
-    # ensure we have a full schema cache in case we missed something above
-    ActiveRecord::Base.connection.data_sources.each do |table|
-      ActiveRecord::Base.connection.schema_cache.add(table)
-    end
-
-    schema_cache = ActiveRecord::Base.connection.schema_cache
-
-    # load up schema cache for all multisite assuming all dbs have
-    # an identical schema
-    RailsMultisite::ConnectionManagement.each_connection do
-      dup_cache = schema_cache.dup
-      # this line is not really needed, but just in case the
-      # underlying implementation changes lets give it a shot
-      dup_cache.connection = nil
-      ActiveRecord::Base.connection.schema_cache = dup_cache
-    end
-
-    # router warm up
-    Rails.application.routes.recognize_path('abc') rescue nil
-
-    # preload discourse version
-    Discourse.git_version
-    Discourse.git_branch
-    Discourse.full_version
+    Discourse.preload_rails!
 
     # V8 does not support forking, make sure all contexts are disposed
     ObjectSpace.each_object(MiniRacer::Context) { |c| c.dispose }
 
     # get rid of rubbish so we don't share it
+    # longer term we will use compact! here
+    GC.start
+    GC.start
     GC.start
 
     initialized = true