Adds sample for creating a database with a version retention
period. Updates sample for creating a backup with a version time.
Updates sample for restoring a backup, printing the backup version time.

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Uses the earliest version time from the database as the version time for
the backup.

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* samples: creates a backup using the current time

Instead of using the earliest version time of the database, uses the
current time (from spanner). If we used the earliest version time of the
database instead we would be creating an empty backup, since the
database we are backing up is not 1 hour old (default version retention
period).

* samples: remove unused variable

Clean up create backup samples by removing unused variable.

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