Improve the performance of reading belongs_to associations

`ActiveRecord::Base#[]` has overhead that was introduced in 4.2. The
`foo["id"]` working with PKs other than ID isn't really a case that we
want to support publicly, but deprecating was painful enough that we
avoid it. `_read_attribute` was introduced as the faster alternative for
use internally. By using that, we can save a lot of overhead. We also
save some overhead by reading the attribute one fewer times in
`stale_state`.

Fixes #18151

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -73,11 +73,11 @@ def increment_counter(counter_cache_name)
 
         # Checks whether record is different to the current target, without loading it
         def different_target?(record)
-          record.id != owner[reflection.foreign_key]
+          record.id != owner._read_attribute(reflection.foreign_key)
         end
 
         def replace_keys(record)
-          owner[reflection.foreign_key] = record[reflection.association_primary_key(record.class)]
+          owner[reflection.foreign_key] = record._read_attribute(reflection.association_primary_key(record.class))
         end
 
         def remove_keys
@@ -85,7 +85,7 @@ def remove_keys
         end
 
         def foreign_key_present?
-          owner[reflection.foreign_key]
+          owner._read_attribute(reflection.foreign_key)
         end
 
         # NOTE - for now, we're only supporting inverse setting from belongs_to back onto
@@ -99,12 +99,13 @@ def target_id
           if options[:primary_key]
             owner.send(reflection.name).try(:id)
           else
-            owner[reflection.foreign_key]
+            owner._read_attribute(reflection.foreign_key)
           end
         end
 
         def stale_state
-          owner[reflection.foreign_key] && owner[reflection.foreign_key].to_s
+          result = owner._read_attribute(reflection.foreign_key)
+          result && result.to_s
         end
     end
   end