Add worker retry and future updates support

The Barbican worker processes need a means to support retrying failed
yet recoverable tasks (such as when remote systems are unavailable) and
for handling updates for long-running order processes such as
certificate generation. This blueprint defines the requirements for
this retry and update processing, and proposes an implementation to add
this feature.

Change-Id: Ifce76a6c6b279111f8476e6804db07c519f59a08
Merge "Add worker retry and future updates support"