Implement wait-for-ready behavior in Python stress and qps client

The clients now block until the channel is in the READY state. This
fixes some test flakiness issues we have had.

src/python/grpcio_tests/tests/qps/benchmark_client.py

@@ -68,12 +68,8 @@ class BenchmarkClient:
     else:
       channel = grpc.insecure_channel(server)
 
-    connected_event = threading.Event()
-    def wait_for_ready(connectivity):
-      if connectivity == grpc.ChannelConnectivity.READY:
-        connected_event.set()
-    channel.subscribe(wait_for_ready, try_to_connect=True)
-    connected_event.wait()
+    # waits for the channel to be ready before we start sending messages
+    grpc.channel_ready_future(channel).result()
 
     if config.payload_config.WhichOneof('payload') == 'simple_params':
       self._generic = False

src/python/grpcio_tests/tests/stress/client.py

@@ -110,10 +110,13 @@ def _get_channel(target, args):
     channel_credentials = grpc.ssl_channel_credentials(
         root_certificates=root_certificates)
     options = (('grpc.ssl_target_name_override', args.server_host_override,),)
-    return grpc.secure_channel(
-        target, channel_credentials, options=options)
+    channel = grpc.secure_channel(target, channel_credentials, options=options)
   else:
-    return grpc.insecure_channel(target)
+    channel = grpc.insecure_channel(target)
+
+  # waits for the channel to be ready before we start sending messages
+  grpc.channel_ready_future(channel).result()
+  return channel
 
 def run_test(args):
   test_cases = _parse_weighted_test_cases(args.test_cases)