Add Base.shards for console-convience

This adds some across-all-shards convience methods for the console:

`Model.shards.find(id)` for finding a record when you're not sure what shard it's on.

`where(nil).shards.to_a` to grab all records across shards 

`where(nil).shards.count` to count across shards

@yadavsaroj @staugaard @zendesk/sustaining