hydra-queue-runner: Improved scheduling

Each jobset now has a "scheduling share" that determines how much of
the build farm's time it is entitled to.  For instance, if a jobset
has 100 shares and the total number of shares of all jobsets is 1000,
it's entitled to 10% of the build farm's time.  When there is a free
build slot for a given system type, the queue runner will select the
jobset that is furthest below its scheduling share over a certain time
window (currently, the last day).  Withing that jobset, it will pick
the build with the highest priority.

So meta.schedulingPriority now only determines the order of builds
within a jobset, not between jobsets.  This makes it much easier to
prioritise one jobset over another (e.g. nixpkgs:trunk over
nixpkgs:stdenv).

src/lib/Hydra/Helper/Nix.pm

@@ -20,7 +20,8 @@ our @EXPORT = qw(
     getMainOutput
     getEvals getMachines
     pathIsInsidePrefix
-    captureStdoutStderr run grab);
+    captureStdoutStderr run grab
+    getTotalShares);
 
 
 sub getHydraHome {
@@ -533,4 +534,12 @@ sub grab {
 }
 
 
+sub getTotalShares {
+    my ($db) = @_;
+    return $db->resultset('Jobsets')->search(
+        { 'project.enabled' => 1, 'me.enabled' => 1 },
+        { join => 'project', select => { sum => 'schedulingshares' }, as => 'sum' })->single->get_column('sum');
+}
+
+
 1;