queue: explicitly declare ordering requirement

This change defaults the ordering requirement of queue items to be off
and only enables it for the build manager. This should make the queries
for getting queueitems significantly faster for every other use case.

buildman/server.py

@@ -163,7 +163,8 @@ class BuilderServer(object):
       logger.debug('Checking for more work for %d active workers',
                    self._lifecycle_manager.num_workers())
 
-      job_item = self._queue.get(processing_time=self._lifecycle_manager.setup_time())
+      job_item = self._queue.get(processing_time=self._lifecycle_manager.setup_time(),
+                                 order_required=True)
       if job_item is None:
         logger.debug('No additional work found. Going to sleep for %s seconds', WORK_CHECK_TIMEOUT)
         continue