Support previously_generated_holdings_files in MFHD transformer tasks

When you run multiple delimited file holdings transformer tasks, you can specify a file, or files, of previously-generated holdings records to include with the records being generated by the current task. If you don't do this for consecutive holdings transformer tasks, the previous task's holdings ids will not be included in the holdings map, which can create problems for item record transformation. The MFHD transformer does not allow this. It should.