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/dispatcher.cc

@@ -31,8 +31,10 @@ void State::makeRunnable(Step::ptr step)
 
 void State::dispatcher()
 {
-    while (true) {
+    printMsg(lvlDebug, "Waiting for the machines parsing to have completed at least once");
+    machinesReadyLock.lock();
 
+    while (true) {
         try {
             printMsg(lvlDebug, "dispatcher woken up");
             nrDispatcherWakeups++;