Automatically retry aborted builds

Aborted builds are now put back on the runnable queue and retried
after a certain time interval (currently 60 seconds for the first
retry, then tripled on each subsequent retry).

src/sql/hydra.sql

@@ -372,6 +372,7 @@ create table CachedCVSInputs (
 );
 
 
+-- FIXME: remove
 create table SystemTypes (
     system        text primary key not null,
     maxConcurrent integer not null default 2