Freeze fragment cache related instrument name.

ActionMailer::Base#instrument_name and
ActionController::Base#instrument_name will be frequently called once
caching is enabled. So it's better to freeze them instead of create new
string on every call.

Also, the instrument name in #instrument_fragment_cache will usually
be "write_fragment.action_controller" or
"read_fragment.action_controller". So freezing them might also gain some
performance improvement. We have done something like this in other places:
https://github.com/rails/rails/blob/master/actionview/lib/action_view/template.rb#L348

actionmailer/lib/action_mailer/base.rb

@@ -972,7 +972,7 @@ module ActionMailer
       end
 
       def instrument_name
-        "action_mailer"
+        "action_mailer".freeze
       end
 
       ActiveSupport.run_load_hooks(:action_mailer, self)

actionpack/lib/abstract_controller/caching/fragments.rb

@@ -136,7 +136,7 @@ module AbstractController
 
       def instrument_fragment_cache(name, key) # :nodoc:
         payload = instrument_payload(key)
-        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", payload) { yield }
+        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}".freeze, payload) { yield }
       end
     end
   end

actionpack/lib/action_controller/caching.rb

@@ -38,7 +38,7 @@ module ActionController
       end
 
       def instrument_name
-        "action_controller"
+        "action_controller".freeze
       end
   end
 end