Use identifier instead of step uid in transfer table

[brotkrueml]

This is a preparation to #6 to smoothen the transition to version 2.0. The step uid is then not available anymore, instead an "identifier" (formerly known as "handle") is available. The transfer table should use the identifier/handle instead. This change should be available in version 1.

• A step identifier is added to the transfer table.
• When processing transfers the identifier/handle is used instead of the step_uid.
• If an identifier is empty the step uid is used instead.