Allow to add a certain string to last run files names.

Problem:

Currently to know when a harvester last ran, we store a file with a timestamp. The file currently has a fixed name. If now the same harvester type runs in parallel but harvests others sources, they read and write to the same last run file.

Proposed Solution: add the functionality to save and load last run information from a given string. that way one can write and load a last run file for every source.