http: fix deadlock in the test case

There was a real chance that httpd.py got stuck blocked on stdio.write
due to the buffer overflow. The very frequent heartbeat was set to flush
the buffer faster. This commit decreases heartbeat frequency but makes
stdio flush manually.

test/app-tap/httpd.py

@@ -97,7 +97,8 @@ def heartbeat():
     try:
         while True:
             sys.stdout.write("heartbeat\n")
-            sleep(1e-3)
+            sys.stdout.flush()
+            sleep(1e-1)
     except IOError:
         sys.exit(1)