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/deploy/build_background_processing_queue.sql

@@ -0,0 +1,9 @@
+-- Deploy guix-data-service:build_background_processing_queue to pg
+
+BEGIN;
+
+CREATE TABLE build_background_processing_queue (
+       build_id integer PRIMARY KEY REFERENCES builds (id)
+);
+
+COMMIT;