Clustering: Serialization failures when upgrading from a Helium journal

I tried to load a journal from a Helium SR2 install into a Lithium install to simulate an upgrade. It failed with serialized version mismatch errors for the UpdateElectionTerm and DeleteEntries classes.

Earlier in Li the static serialVersionUID field was aded to these classes (set to 1). We can't do this as it breaks backwards compatibility.

The DeleteEntries inner class in RaftActor was actually deprecated in favor of a new class in the messages package. We should do the same for the UpdateElectionTerm inner class. We can remove the deprecated inner classes in a later release.