hydra-evaluator: add a 'ONE_AT_A_TIME' evaluator style

In the past, jobsets which are automatically evaluated are evaluated
regularly, on a schedule. This schedule means a new evaluation is
created every checkInterval seconds (assuming something changed.)

This model works well for architectures where our build farm can
easily keep up with demand.

This commit adds a new type of evaluation, called ONE_AT_A_TIME, which
only schedules a new evaluation if the previous evaluation of the
jobset has no unfinished builds.

This model of evaluation lets us have 'low-tier' architectures.

For example, we could now have a jobset for ARMv7l builds, where
the buildfarm only has a single, underpowered ARMv7l builder.
Configuring that jobset as ONE_AT_A_TIME will create an evaluation
and then won't schedule another evaluation until every job of
the existing evaluation is complete.

This way, the cache will have a complete collection of pre-built
software for some commits, but the underpowered architecture will
never become backlogged in ancient revisions.

src/root/edit-jobset.tt

@@ -68,6 +68,7 @@
           <input type="hidden" name="enabled" value="[% jobset.enabled %]" />
           <button type="button" class="btn" value="1">Enabled</button>
           <button type="button" class="btn" value="2">One-shot</button>
+          <button type="button" class="btn" value="3">One-at-a-time</button>
           <button type="button" class="btn" value="0">Disabled</button>
         </div>
       </div>