Janitor fails to deserialize persistent states due to extra '_id' attribute

[radek-macha]

15:54:42.881 [Thread-7] WARN  c.c.m.d.b.o.janitor.TrashSeeker - Cannot load started task context list for cleanup
cz.cuni.mff.d3s.been.util.JsonException: Error when unmarshalling collection of PersistentContextState
        at cz.cuni.mff.d3s.been.util.JSONUtils.deserialize(JSONUtils.java:125) ~[been.jar:na]
        at cz.cuni.mff.d3s.been.objectrepository.janitor.TrashSeeker.getStartedContextsPastDue(TrashSeeker.java:193) ~[been.jar:na]
        at cz.cuni.mff.d3s.been.objectrepository.janitor.Janitor.loadMore(Janitor.java:160) [been.jar:na]
        at cz.cuni.mff.d3s.been.objectrepository.janitor.Janitor.doSweep(Janitor.java:140) [been.jar:na]
        at cz.cuni.mff.d3s.been.objectrepository.janitor.Janitor.doRun(Janitor.java:122) [been.jar:na]
        at cz.cuni.mff.d3s.been.objectrepository.janitor.Janitor.run(Janitor.java:89) [been.jar:na]
Caused by: org.codehaus.jackson.map.exc.UnrecognizedPropertyException: Unrecognized field "_id" (Class cz.cuni.mff.d3s.been.persistence.task.PersistentContextState), not marked as ignorable
 at [Source: java.io.StringReader@6036c24b; line: 1, column: 12] (through reference chain: cz.cuni.mff.d3s.been.persistence.task.PersistentContextState["_id"])
        at org.codehaus.jackson.map.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:53) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.StdDeserializationContext.unknownFieldException(StdDeserializationContext.java:267) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.std.StdDeserializer.reportUnknownProperty(StdDeserializer.java:673) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:659) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.BeanDeserializer.handleUnknownProperty(BeanDeserializer.java:1365) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.BeanDeserializer._handleUnknown(BeanDeserializer.java:725) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:703) ~[been.jar:na]
        at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:580) ~[been.jar:na]
        at org.codehaus.jackson.map.ObjectReader._bindAndClose(ObjectReader.java:768) ~[been.jar:na]
        at org.codehaus.jackson.map.ObjectReader.readValue(ObjectReader.java:460) ~[been.jar:na]
        at cz.cuni.mff.d3s.been.util.JSONUtils.deserialize(JSONUtils.java:122) ~[been.jar:na]
        ... 5 common frames omitted
16:24:42.892 [Thread-7] WARN  c.c.m.d.b.o.janitor.TrashSeeker - Cannot load started task list for zombie detection