HighestIdInterface

Defines an interface for destination and ID maps which track a highest ID.

When implemented by destination plugins, getHighestId() should return the highest ID of the destination entity type that exists in the system. So, for example, the entity:node plugin should return the highest node ID that exists, regardless of whether it was created by a migration.

When implemented by an ID map, getHighestId() should return the highest migrated ID of the destination entity type.