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Added support for "now" and "startup" values to the before option when
using ZConfig. The "now" value indicates that the before storage should
provide a view of the base storage as of the time the storage is created.
The "startup" value indicates that the before storage should provide a
view of the base stoage as of process startup time. The later is
especially useful when setting up more than once before storage in a
single application, as it allows you to arrange that all of the
storages provide consistent views without having to specify a time.