papi: let async calls return context

papi: let async calls return context

Callback receives "reply" messages containing context,
but previously there was no easy way
to get the automatically generated context value
of the originally sent "command" message.

With this, the caller can store the contexts,
so the callback knows which command got replied to.

Type: feature

Change-Id: I58ca812d20b03916f74096c396126710115a747c
Signed-off-by: Vratko Polak <vrpolak@cisco.com>