Reliably perform the background processing for build events

Insert the build ids to a table and remove the ids once processed. If the data
service is restarted and the in memory queue is lost, process all the items
from the database queue.

sqitch/revert/build_background_processing_queue.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:build_background_processing_queue from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;