Dealt with race condition detector warnings:

* stopped obvious race condition with replacing handler in
  ServingLargeResponses test
* lowered probability of other race conditions with test code
  while test server is yet activating
* fixed errmessage in Shutdown
Restored code when shutdown needs an exchange...

Clients sents NULL-packet to server which helps to avoid
timeout. Timeout is still possible to encounter.
Shutdown will likely report error for those cases.