Allow to resume partition processor not only from the latest snapshot

[tillrohrmann]

In case a snapshot gets corrupted and users still have a snapshot available which is resumable (log has been trimmed beyond the applied index), it would be great to have a way to configure the system to resume from it. At the moment, we only try to resume from the latest snapshot. A simple way could be an utility which allows one to drop newer snapshots and rewrite the latest.json file.

cc @pcholakov

[pcholakov]

Makes sense! This would be nice in conjunction with the ability to retain the "last N" snapshots together with a feature to report only the Nth most recent snapshot's LSN as the archived LSN (to delay automatic log trimming).