Version ID. This is a unique number representing this particular entry.
Every update changes the version ID (unless the update doesn't modify
anything, in which case it's permissible for version ID to stay the
same). Services are free to interpret this string in the most convenient
way. Some services may choose to use a monotonically increasing sequence
of version IDs. Other services may compute a hash of entry properties and
use that.
This property is only used for services to communicate the current
version ID back to the servlet. It is NOT set when entries are parsed
(either from requests or from arbitrary XML).