Checksums

I'm wondering if this can't lead to any problems in combination with https://github.com/Incubaid/arakoon/pull/502/files#diff-8312f0aa9ee0fb85222835bef6a0c9dcR164 where the lease_start is updated (and thus changes the serialization of the value) ?
Actually writing the lease start to disk here is (I think) the wrong thing to do.
I think this should ideally not be serialized or a dummy value - which should always be the same - should be serialized.