Machine status: Don't show removed machines anymore

This requires a sequential scan on the BuildSteps table, which by now
takes > 8s.

src/lib/Hydra/Controller/Root.pm

@@ -75,12 +75,18 @@ sub status :Local {
 sub machines :Local Args(0) {
     my ($self, $c) = @_;
     my $machines = getMachines;
-    my $idles = $c->model('DB::BuildSteps')->search(
-            { stoptime => { '!=', undef } },
-            { select => [ 'machine', { max => 'stoptime', -as => 'max_stoptime' }], group_by => "machine" });
-    while (my $idle = $idles->next) {
-        ${$machines}{$idle->machine}{'idle'} = $idle->get_column('max_stoptime');
+
+    # Add entry for localhost.
+    ${$machines}{''} //= {};
+
+    # Get the last finished build step for each machine.
+    foreach my $m (keys %{$machines}) {
+        my $idle = $c->model('DB::BuildSteps')->find(
+            { machine => "$m", stoptime => { '!=', undef } },
+            { order_by => 'stoptime desc', rows => 1 });
+        ${$machines}{$m}{'idle'} = $idle ? $idle->stoptime : 0;
     }
+    
     $c->stash->{machines} = $machines;
     $c->stash->{steps} = [ $c->model('DB::BuildSteps')->search(
         { finished => 0, 'me.busy' => 1, 'build.busy' => 1, },