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/sql/hydra.sql

@@ -61,7 +61,7 @@ create table Jobsets (
     errorTime     integer, -- timestamp associated with errorMsg
     lastCheckedTime integer, -- last time the evaluator looked at this jobset
     triggerTime   integer, -- set if we were triggered by a push event
-    enabled       integer not null default 1, -- 0 = disabled, 1 = enabled, 2 = one-shot
+    enabled       integer not null default 1, -- 0 = disabled, 1 = enabled, 2 = one-shot, 3 = one-at-a-time
     enableEmail   integer not null default 1,
     hidden        integer not null default 0,
     emailOverride text not null,