Clear AR connections before starting Sidekiq

This should ensure that connections obtained before starting Sidekiq are
not leaked, leading to connection timeouts.

Fixes gitlab-com/infrastructure#1139

changelogs/unreleased/clear-connections-before-starting-sidekiq.yml

+---
+title: Clear ActiveRecord connections before starting Sidekiq
+merge_request:
+author:

config/initializers/sidekiq.rb

@@ -19,6 +19,12 @@ Sidekiq.configure_server do |config|
     chain.add Gitlab::SidekiqStatus::ClientMiddleware
   end
 
+  config.on :startup do
+    # Clear any connections that might have been obtained before starting
+    # Sidekiq (e.g. in an initializer).
+    ActiveRecord::Base.clear_all_connections!
+  end
+
   # Sidekiq-cron: load recurring jobs from gitlab.yml
   # UGLY Hack to get nested hash from settingslogic
   cron_jobs = JSON.parse(Gitlab.config.cron_jobs.to_json)