Add a latest_build_status table

This will avoid many queries trying to figure out what the latest build status
is, which will hopefuly simplify queries as well as improving performance.

sqitch/deploy/create_latest_build_status.sql

@@ -0,0 +1,27 @@
+-- Deploy guix-data-service:create_latest_build_status to pg
+
+BEGIN;
+
+CREATE TABLE latest_build_status (
+    build_id integer PRIMARY KEY NOT NULL REFERENCES builds(id),
+    "timestamp" timestamp without time zone DEFAULT clock_timestamp(),
+    status guix_data_service.buildstatus NOT NULL
+);
+
+INSERT INTO latest_build_status
+SELECT DISTINCT build_id,
+                first_value(timestamp) OVER rows_for_build AS timestamp,
+                first_value(status) OVER rows_for_build AS status
+FROM build_status
+WINDOW rows_for_build AS (
+  PARTITION BY build_id
+  ORDER BY
+    timestamp DESC,
+    CASE WHEN status = 'scheduled' THEN -2
+         WHEN status = 'started' THEN -1
+         ELSE 0
+    END DESC
+    RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
+);
+
+COMMIT;