Rudder plugins » Change validation

Since we don't have an understanding of the size of event log or existing change request with the old format, we will take an iterative migration solution:
- for event logs, just be able to read them: the one with the old format will tend to not be accessed after time (and if we still need, just log).
- for change request, read both new and old format, write only new.

It may have as a consequence that we need to keep forever that format, but eventlogs are not meant to be rewritten (perhaps a "cold storage" solution for them would be more appropriate, and migration would be done off line on the cold storage only)