Let Builds.timestamp refer to the time the build was added

Previously, for scheduled builds, "timestamp" contained the time the
build was added to the queue, while for finished builds, it was the
time the build finished.  Now it's always the former.

src/script/hydra-build

@@ -54,7 +54,6 @@ sub failDependents {
             $d->update(
                 { finished => 1
                 , logfile => ''
-                , timestamp => time # !!! Why change the timestamp?
                 , iscachedbuild => 0
                 , buildstatus => $drvPath eq $d->drvpath ? 1 : 2
                 , starttime => $time
@@ -307,7 +306,6 @@ sub doBuild {
             , busy => 0
             , locker => ''
             , logfile => ''
-            , timestamp => time # !!! Why change the timestamp?
             , iscachedbuild => $isCachedBuild
             , buildstatus => $buildStatus
             , starttime => $startTime

src/script/hydra-queue-runner

@@ -100,7 +100,7 @@ sub checkBuilds {
             # Select the highest-priority builds to start.
             my @builds = $extraAllowed == 0 ? () : $db->resultset('Builds')->search(
                 { finished => 0, busy => 0, system => $system->system, enabled => 1 },
-                { join => ['project'], order_by => ["priority DESC", "timestamp"],
+                { join => ['project'], order_by => ["priority DESC", "id"],
                   rows => $extraAllowed });
 
             if (scalar(@builds) > 0) {