Switch build queue limiter query to use total number of alive jobs

This is slightly more accurate and, not being based on time, will work better under MySQL

data/queue.py

@@ -74,18 +74,20 @@ class WorkQueue(object):
             ._available_jobs(now, name_match_query)
             .where(~(QueueItem.queue_name << running_query)))
 
-  def num_available_jobs(self, canonical_name_list):
+  def num_alive_jobs(self, canonical_name_list):
     """
-    Returns the number of available queue items with a given prefix.
+    Returns the number of alive queue items with a given prefix.
     """
     def strip_slash(name):
       return name.lstrip('/')
     canonical_name_list = map(strip_slash, canonical_name_list)
+    canonical_name_query = '/'.join([self._queue_name] + canonical_name_list) + '%'
 
-    available = self._available_jobs(datetime.utcnow(),
-                                     '/'.join([self._queue_name] + canonical_name_list) + '%')
-
-    return available.count()
+    return (QueueItem
+            .select()
+            .where(QueueItem.queue_name ** canonical_name_query)
+            .where(QueueItem.retries_remaining > 0)
+            .count())
 
   def num_available_jobs_between(self, available_min_time, available_max_time, canonical_name_list):
     """