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/libhydra/db.hh

@@ -21,18 +21,23 @@ struct Connection : pqxx::connection
 };
 
 
-struct receiver : public pqxx::notification_receiver
+class receiver : public pqxx::notification_receiver
 {
-    bool status = false;
+    std::experimental::optional<std::string> status;
+
+public:
+
     receiver(pqxx::connection_base & c, const std::string & channel)
         : pqxx::notification_receiver(c, channel) { }
+
     void operator() (const std::string & payload, int pid) override
     {
-        status = true;
+        status = payload;
     };
-    bool get() {
-        bool b = status;
-        status = false;
-        return b;
+
+    std::experimental::optional<std::string> get() {
+        auto s = status;
+        status = std::experimental::nullopt;
+        return s;
     }
 };