Aggregations: Do not sort histogram buckets on shards.

Histogram do not perform any selection of the buckets at the shard level so it
is useless to sort buckets there given that we are going to sort again on the
coordinating node once buckets with the same key have been merged.
I wonder if the entire function could be simplified to the form:

```
current = min
while (current <= max && iter.hasNext()) {
    // get next bucket
    // loop current up to nextBucket
   // set current to nextBucket rounded
}

while (current <= max) {
  // handles both empty case and trailing case
}
```