Add support for incrementally tracking blocked builds

This will hopefully provide a less expensive way of finding out if a scheduled
build is probably blocked by other builds failing or being canceled.

By working this out when the build events are recieved, it should be more
feasible to include information about whether builds are likely blocked or not
in various places (e.g. revision comparisons).

sqitch/verify/blocked_builds.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:blocked_builds on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;