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/lib/Hydra/Controller/Jobset.pm

@@ -226,7 +226,7 @@ sub updateJobset {
     my ($nixExprPath, $nixExprInput) = nixExprPathFromParams $c;
 
     my $enabled = int($c->stash->{params}->{enabled});
-    die if $enabled < 0 || $enabled > 2;
+    die if $enabled < 0 || $enabled > 3;
 
     my $shares = int($c->stash->{params}->{schedulingshares} // 1);
     error($c, "The number of scheduling shares must be positive.") if $shares <= 0;