Fix a race that can cause hydra-queue-runner to ignore newly added builds

As @dtzWill discovered, with the concurrent hydra-evaluator, there can
be multiple active transactions adding builds to the database. As a
result, builds can become visible in a non-monotonically increasing
order, breaking the queue monitor's assumption that build IDs only go
up.

The fix is to have hydra-eval-jobset provide the lowest build ID it
just added in the builds_added notification, and have the queue
monitor check from there.

Fixes #496.

src/hydra-queue-runner/queue-monitor.cc

@@ -48,8 +48,10 @@ void State::queueMonitorLoop()
         } else
             conn->get_notifs();
 
-        if (buildsAdded.get())
+        if (auto lowestId = buildsAdded.get()) {
+            lastBuildId = std::stoi(*lowestId) - 1;
             printMsg(lvlTalkative, "got notification: new builds added to the queue");
+        }
         if (buildsRestarted.get()) {
             printMsg(lvlTalkative, "got notification: builds restarted");
             lastBuildId = 0; // check all builds
@@ -75,7 +77,7 @@ struct PreviousFailure : public std::exception {
 bool State::getQueuedBuilds(Connection & conn,
     ref<Store> destStore, unsigned int & lastBuildId)
 {
-    printMsg(lvlInfo, format("checking the queue for builds > %1%...") % lastBuildId);
+    printInfo("checking the queue for builds > %d...", lastBuildId);
 
     /* Grab the queued builds from the database, but don't process
        them yet (since we don't want a long-running transaction). */