Revert "Aggregations: Do not sort buckets on the shard-level."

This reverts commit 5f26e7e86d4d4d66af6c0f76a12f9a7a35f3478f.
Aggregations: reduce histogram buckets on the fly using a priority queue.

This commit makes histogram reduction a bit cleaner by expecting buckets
returned from shards to be sorted by key and merging them on-the-fly on the
coordinating node using a priority queue.