Reduce UPDATE queries when moving between import states on projects

By changing our logic from `before_transition` to `after_transition`
blocks and removing extra calls to `update` or `save` we can do all
changes in a single UPDATE query (currently each transition change
from started -> failed and started -> finished runs two UPDATE
queries each)

CHANGELOG-EE

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.12.0 (Unreleased)
+  - Reduce UPDATE queries when moving between import states on projects
   - [ES] Instrument Elasticsearch::Git::Repository
   - [ES] Instrument other Gitlab::Elastic classes
 

app/models/project.rb

@@ -237,12 +237,11 @@ class Project < ActiveRecord::Base
 
     after_transition any => :finished, do: :reset_cache_and_import_attrs
 
-    after_transition started: :finished do |project, transaction|
+    before_transition started: :finished do |project, transaction|
       if project.mirror?
         timestamp = DateTime.now
         project.mirror_last_update_at = timestamp
         project.mirror_last_successful_update_at = timestamp
-        project.save
       end
 
       if current_application_settings.elasticsearch_indexing?
@@ -250,10 +249,8 @@ class Project < ActiveRecord::Base
       end
     end
 
-    after_transition started: :failed do |project, transaction|
-      if project.mirror?
-        project.update(mirror_last_update_at: DateTime.now)
-      end
+    before_transition started: :failed do |project, transaction|
+      project.mirror_last_update_at = DateTime.now if project.mirror?
     end
   end