hydra-queue-runner: don't dispatch until the machines parser has completed one run

Periodically, I have seen tests fail because of out of order queue runner behavior:

    checking the queue for builds > 0...
    loading build 1 (tests:basic:empty_dir)
    aborting unsupported build step '...-empty-dir.drv' (type 'x86_64-linux')
    marking build 1 as failed
    adding new machine ‘localhost’

This patch should prevent the dispatcher from running before any machines are
made available.

src/hydra-queue-runner/state.hh

@@ -342,6 +342,7 @@ private:
     nix::Pool<Connection> dbPool;
 
     /* The build machines. */
+    std::mutex machinesReadyLock;
     typedef std::map<std::string, Machine::ptr> Machines;
     nix::Sync<Machines> machines; // FIXME: use atomic_shared_ptr