Introduce some stress tests. (#106)

* Retry first connection to redis in db_connect.

* Declare usleep.

* Formatting.

* Introduce some stress tests.

lib/python/ray/services.py

@@ -66,6 +66,9 @@ def cleanup():
     print("Ray did not shut down properly.")
   all_processes = []
 
+def all_processes_alive():
+  return all([p.poll() is None for p in all_processes])
+
 def start_redis(num_retries=20, cleanup=True):
   """Start a Redis server.