Store tombstone in parent on update

[will2041]

Reference lines -

honu/versions.go

Lines 56 to 61 in d13a85a

	if meta.Version != nil && !meta.Version.IsZero() {
	meta.Version.Parent = &pb.Version{
	Pid: meta.Version.Pid,
	Version: meta.Version.Version,
	Region: meta.Version.Region,
	}

Implementation does not copy tombstone data into the parent version. This creates a problem when backtracking through history. A deleted object will not have a stored tombstone transition and can only be identified by looking at nil data.

Storing the tombstone allows for a reconstruction of the history of the object, which is necessary when defining expected behavior for users.

Line 61 just needs:

Tombstone: meta.Version.Tombstone,

Bonus points for adding a complete clone() method for taking care of Version copying. :D