Use a List of shards per shard ID rather than a set.

The shards in the set are mutated after they are added to the
set such that the hashcode doesn't fit anymore. For this reason
this used an identity hashset before but the downside of this is
that the iteration order is not deterministic. We can just use a list
since shard removal is a very rare action and the size of the list is
very small such that iteration is fast.