No need to do this via an allocation-heavy `Stream`, we can just put the
objects straight into an array, sort them in-place, and keep hold of the
array to avoid having to allocate anything on the next iteration.

Also slims down `BY_DESCENDING_SHARD_ID`: it's always sorting the same
index so we don't need to look at `ShardId#index` in the comparison, nor
do we really need multiple layers of vtable lookups, we can just compare
the shard IDs directly.

Relates elastic#128021