Dynamic, date based, tag (zone) ranges

Method to create dynamic, date based, tag ranges for shard tags. E.g. Within the past 7 days => "recent", greater than 7 days => "archive".

Currently, this can be done with a cron job to continually adjust tag ranges, however, this is only becoming more popular and the cron job option adds another level of complexity to deployments.

e.g. all recent photo albums are on "recent" tagged shards (where the ratio of data size to RAM is much lower or the drive is SSD)

sh.addTagRange("photos.albums", 

{ "date_uploaded" : { $currentDate : true }  }, 

{ "date_uploaded" : { $currentDate : -7 }  }, 

"recent")