Returns the visibility token of the underlying NodeStore. A 'visibility
token' is an opaque String that can be used to verify if changes done on
one NodeStore are visible on another NodeStore of the same cluster. This
can be achieved by generating such a visibility token on the source
NodeStore, passing it on to the target NodeStore (by whatever means) and
checking for visibility on that target NodeStore.
The visibility check returns true if the target NodeStore sees at least
all the changes that the source NodeStore saw at time of visibility token
generation. Once a visibility token is visible on a particular NodeStore
it will always return true ever after. This also implies that the
visibility check can only state whether at least all source changes are
visible on the target and that it is independent of any further
modifications.
When source and target NodeStore are identical, the visibility check is
expected to return true, immediately. This is based on the assumption
that with a session.refresh() on that NodeStore you'll always get the
latest changes applied by any other session locally.
Visibility tokens are meant to be lightweight and are not expected to be
persisted by the implementor. Nevertheless they should survive their
validity in the case of crashes of the source and/or the target instance.