Store same app-service secret fails on second request

[cherrycl]

[Reproduce Steps]

1. Deploy edgex with the docker-compose file.
2. Send the following API twice

POST  http://localhost:59704/api/v2/secret
Request body:
{
   "apiVersion":"v2",
   "path":"MyPath",
   "secretData":[
      {
         "key":"MySecretKey",
         "value":"MySecretValue"
      }
   ]
}

[Actual result]
API returns status code 201 at the first request, but fail with 503 on second request.

[lenny-goodell]

@FelixTing , this is also caused by your #1046.
In this case nothing is waiting on the channel for the secrets to be pushed. So the second attempt get blocked on write to the channel.