afb-xreq: Forbids (un)subscribes after reply
Allowing to subscribe to a request that was replied
was possible and lead to problem revealed by issues
SPEC-2542 and SPEC-2599 (these issues are more related
to processing unordered incoming messages).
The choice was
- fix the bug and authorize (un)subscribe after reply
- forbids to (un)subscribe after reply
Second solution was chosen for its simplicity and
its networking efficiency.
Bug-AGL: SPEC-2542
Bug-AGL: SPEC-2599
Change-Id: I09f48b760b1fd6f70d42b80df3c8053696c45966
Signed-off-by: Jose Bollo <jose.bollo@iot.bzh>