public abstract class PriorityComparator
A comparator that compares ShardRouting based on it's indexes priority (index.priority),
it's creation date (index.creation_date), or eventually by it's index name in reverse order.
We try to recover first shards from an index with the highest priority, if that's the same
we try to compare the timestamp the index is created and pick the newer first (time-based indices,
here the newer indices matter more). If even that is the same, we compare the index name which is useful
if the date is baked into the index name. ie logstash-2015.05.03.