Trigger snapshots on legacy persisted entries

We have migrated proxies, hence we should be purging them from journal
if we encounter them during recovery. Introduce a marker interface for
that and subclasses which implement it.

JIRA: CONTROLLER-2064
Change-Id: Ie16b1247306d64580df977bece70a94eb3187cbf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>