Add an option for the swift-manage-shard-ranges repair tool to ignore
overlaps where the overlapping shards appear to be recently created,
since this may indicate that they are shards of the parent with
which they overlap.
The new option is --min-shard-age with a default of 14400 seconds.
Change-Id: Ib82219a506732303a1157c2c9e1c452b4a56061b