afb-stub-ws: Allow unordered process of messages

Requests are already sequenced on need (noconcurrency
flag of bindings). Sequencing messages until their
completion has a side effect when dealing with events
and synchronous calls: event -> call -> reply locked
until event completion (see SPEC-2219).

This change could be reverted if events are converted
to jobs (see SPEC-2215).

Bug-AGL: SPEC-2219

Change-Id: Ia577bc54ba38291817fd5ed3e3dda1b67b2818b7
Signed-off-by: José Bollo <jose.bollo@iot.bzh>