Ability to delete series on a series-by-series basis

[e-dard]

Currently the tsdb.Shard API allows a caller to delete a series across a range of time. For some use-cases we would like to be able to decide whether to delete a series according to some property of the series key.

In this case we could provide another method on the Shard to pass in both a range of time and a predicate, where the predicate would check each series key and decide if it should be deleted or not.