clear active connections after each job

in a sharded environment, this helps to clear out idle connections

this is similar to what
ActiveRecord::ConnectionAdapters::ConnectionManagement middleware
does after each web request

test plan:
 * run some jobs
 * they should not fail

Change-Id: I431205ff1e9197940bddf4028d3c4c7662a74056
Reviewed-on: https://gerrit.instructure.com/24285
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

vendor/plugins/delayed_job/lib/delayed/backend/base.rb

@@ -225,8 +225,12 @@ module Delayed
       # Moved into its own method so that new_relic can trace it.
       def invoke_job
         Delayed::Job.in_delayed_job = true
-        payload_object.perform
-        Delayed::Job.in_delayed_job = false
+        begin
+          payload_object.perform
+        ensure
+          Delayed::Job.in_delayed_job = false
+          ActiveRecord::Base.clear_active_connections! unless Rails.env.test?
+        end
       end
 
       def batch?