Fix Status serialization with cleanupCallback as Closure

Fix Status serialization with cleanupCallback as Closure

We already have Status objects throw away the cleanupCallback in
__wakeup. Why not also implement __sleep to not save it in the first
place?

The immediate motivation here is that cleanupCallback could be a
Closure, which causes an exception when serialization is attempted (e.g.
when trying to save the Status if an AssembleUploadChunks job fails
verification).

Note this leaves __wakeup intact, in case of old objects where
cleanupCallback is still present in the serialized representation.

Change-Id: I3aa756cd4eb5553ce0b95e7088b929b2f529abfe