Factorize common code for handling requests

Factorize common code for handling requests

The common code for session handling is
shared using struct afb_xreq. At the moment
only hreq leverages the new feature.

The objective is double: make the work of
writing new internal requests more easy and
prepare to check permissions.

Change-Id: If3ca311d68c2d8c427d1125f31a2704b150c2c94
Signed-off-by: José Bollo <jose.bollo@iot.bzh>