Change the completionhandler to accept Res

These modifications are from WIP:Redfish:Query parameters:Only
(https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/47474). It will
be used in future CLs for Query Parameters.

The code changed the completion handle to accept Res to be able to
recall handle with a new Response object.
AsyncResp owns a new res, so there is no need to pass in a res.

Also fixed a self-move assignment bug.

Context:
Originally submitted:
https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/480020

Reveted here:
https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/48880

Because of failures here:
https://gerrit.openbmc-project.xyz/c/openbmc/openbmc/+/48864



Tested:
1. Romulus QEMU + Robot tests; all passed
2. Use scripts/websocket_test.py to test websockets. It is still work correctly.
3. Tested in real hardware; no new validator errors; tested both
   authless, session, and basic auth.
4. Hacked codes to return 500 errors on certain resource; response is
   expected;
5. Tested Eventing, the push style one (not SSE which is still under
   review), worked as expected.
6. Tested 404 errors; response is expected.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Signed-off-by: John Edward Broadbent <jebr@google.com>
Change-Id: I52adb174476e0f6656335baa6657456752a031be