SNMP interface RRD migrator should create and clean up backups interface-wise

The SnmpInterfaceRrdMigratorOnline class currently creates ZIP backups of all candidate interfaces across all nodes before starting to do any work. It does not clean up any of these backups until all its work is done.

For very large systems with many interfaces and extended RRD data retention configurations, this approach could result in the filesystem filling up before the work is completed. If the task instead created the backups for each candidate interface just before starting the merge work for that interface, and cleaned them up immediately after successfully doing the merge work for that interface, this hazard would be mitigated.