BURs: update posts in parallel.

When processing an alias, rename, implication, mass update, or nuke,
update the posts in parallel. This means that if we alias foo to bar,
for example, then we use four processes at once to retag the posts from
foo to bar.

This doesn't mean that if we have two aliases in a BUR, we process both
aliases in parallel. It simply means that when processing an alias, we
update the posts in parallel for that alias.

test/unit/bulk_update_request_test.rb

@@ -266,6 +266,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         should "update the tags" do
           assert_equal("bar", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "be case-sensitive" do
@@ -288,6 +289,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         should "rename the tags" do
           assert_equal("bar blah", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "move the tag's artist entry and wiki page" do
@@ -351,6 +353,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
           assert_equal("foo", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "remove implications" do
@@ -370,6 +373,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
           assert_equal([], @pool.post_ids)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @pool.versions.last.updater)
         end
       end
 
@@ -514,6 +518,10 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         assert_equal("approved", @bur.reload.status)
       end
 
+      should "update the post as DanbooruBot" do
+        assert_equal(User.system, @post.versions.last.updater)
+      end
+
       should "set the BUR as failed if there is an unexpected error during processing" do
         @bur = create(:bulk_update_request, script: "alias one -> two")
         TagAlias.any_instance.stubs(:process!).raises(RuntimeError.new("oh no"))