ensure ExternalFeedAggregator doesn't choke through pgbouncer

refs CNVS-15875

Change-Id: I2ac0b7ddf350b08486ad0ea4cf03e3715cc1f39a
Reviewed-on: https://gerrit.instructure.com/43182
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/external_feed_aggregator.rb

@@ -31,10 +31,12 @@ class ExternalFeedAggregator
   
   def process
     Shackles.activate(:slave) do
-      ExternalFeed.to_be_polled.find_each do |feed|
-        next if !feed.context || feed.context.root_account.deleted?
-        Shackles.activate(:master) do
-          process_feed(feed)
+      ExternalFeed.transaction do
+        ExternalFeed.to_be_polled.find_each do |feed|
+          next if !feed.context || feed.context.root_account.deleted?
+          Shackles.activate(:master) do
+            process_feed(feed)
+          end
         end
       end
     end