messaging: stop using RequestContextSerializer
messaging: stop using RequestContextSerializer

This has been deprecated from oslo.messaging, let's explicitly use it.

Change-Id: I52febba468bbdbaa4e501b9bb61374fa6ab45e9e

openstack/ceilometer,idegtiarov/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer,ityaptin/ceilometer,openstack/ceilometer