Remove obsolete Builds columns and provide accurate "Running builds"

This removes the "busy", "locker" and "logfile" columns, which are no
longer used by the queue runner. The "Running builds" page now only
shows builds that have an active build step.

src/sql/upgrade-43.sql

@@ -0,0 +1,3 @@
+alter table Builds drop column busy, drop column locker, drop column logfile;
+
+drop index IndexBuildsOnFinishedBusy;