Add configurable limits for number of builds allowed under a namespace

We also support that limit being increased automatically once a successful billing charge has gone through

data/queue.py

@@ -74,6 +74,19 @@ class WorkQueue(object):
             ._available_jobs(now, name_match_query)
             .where(~(QueueItem.queue_name << running_query)))
 
+  def num_available_jobs(self, canonical_name_list):
+    """
+    Returns the number of available queue items with a given prefix.
+    """
+    def strip_slash(name):
+      return name.lstrip('/')
+    canonical_name_list = map(strip_slash, canonical_name_list)
+
+    available = self._available_jobs(datetime.utcnow(),
+                                     '/'.join([self._queue_name] + canonical_name_list) + '%')
+
+    return available.count()
+
   def num_available_jobs_between(self, available_min_time, available_max_time, canonical_name_list):
     """
     Returns the number of available queue items with a given prefix, between the two provided times.