afb-xreq: Forbids (un)subscribes after reply

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>