* Allow scheduled builds to be cancelled. They're not removed from

the database, just marked as cancelled, because otherwise the
  scheduler would just add them again.

src/sql/hydra.sql

@@ -61,6 +61,7 @@ create table BuildResultInfo (
     --   1 = build of this derivation failed
     --   2 = build of some dependency failed
     --   3 = other failure (see errorMsg)
+    --   4 = build cancelled (removed from queue; never built)
     buildStatus   integer,
 
     errorMsg      text, -- error message in case of a Nix failure