Call the requests callbacks only once per request

Call the requests callbacks only once per request
Message creation was like that already, but now we call the response or
error callback for the request only once. The cache does what it needs
there and returns, in the case of a successful response, the message
that will be sent to the client.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/evas@73981 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33