BURs: lock posts when updating tags during bulk updates.

Fixes updates sometimes getting clobbered when multiple aliases or mass
updates tried to update the same post at the same time.

app/jobs/tag_batch_change_job.rb

@@ -28,9 +28,10 @@ class TagBatchChangeJob < ApplicationJob
 
   def migrate_posts(normalized_antecedent, normalized_consequent)
     ::Post.tag_match(normalized_antecedent.join(" ")).find_each do |post|
-      post.reload
-      tags = (post.tag_array - normalized_antecedent + normalized_consequent).join(" ")
-      post.update(tag_string: tags)
+      post.with_lock do
+        tags = (post.tag_array - normalized_antecedent + normalized_consequent).join(" ")
+        post.update(tag_string: tags)
+      end
     end
   end