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/root/jobset.tt

@@ -1,5 +1,6 @@
 [% WRAPPER layout.tt title="Jobset $project.name:$jobset.name" %]
 [% PROCESS common.tt %]
+[% USE format %]
 
 
 [% BLOCK renderJobsetInput %]
@@ -121,6 +122,10 @@
         <th>Check interval:</th>
         <td>[% jobset.checkinterval || "<em>disabled</em>"  %]</td>
       </tr>
+      <tr>
+        <th>Scheduling shares:</th>
+        <td>[% jobset.schedulingshares %] [% IF totalShares %] ([% f = format("%.2f"); f(jobset.schedulingshares / totalShares * 100) %]% out of [% totalShares %] shares)[% END %]</td>
+      </tr>
       <tr>
         <th>Enable email notification:</th>
         <td>[% jobset.enableemail ? "Yes" : "No" %]</td>