Sync rpc and notifier from oslo-incubator.

This patch syncs the latest changes to rpc and notifier from
oslo-incubator.  The most significant changes are for adding support for
a message envelope for all messages, including notifications, sent via
rpc.

Related oslo-incubator reviews:

    https://review.openstack.org/#/c/17554/
    https://review.openstack.org/#/c/18057/

Note that a new notifier module was added: rpc_notifier2, which when
used, sends notifications out with a message envelope.

DocImpact.

Implements bp version-rpc-messages.

Change-Id: I495c224d0c502086ce3db6c078d48829b8d913a6
Merge "Sync rpc and notifier from oslo-incubator."