Extract Engine calls used in logExistingProcessInstanceInfos

[BenFuernrohr]

In ProcessInstanceMigrator beginning in line 119 two calls to the ProcessEngine happen.

These calls are to be extracted in two Interfaces:

• "LoadProcessInstances" containing a method loading a List of Process Instances. A Process Instance Model class containing the necessary fields is to be implemented and used as a return value.
• "LoadVersionTag", containing a method loading the versionTag of a given Process Instance (identified by ID).

A default Implementation for both interfaces is to be provided (equal to the existing implementation in ProcessInstanceMigrator) und made the default in ProcessInstanceMigratorBuilder.

Documentation is to be adjusted where necessary.