Add an API to support splitting a given range into shards individually

Writing a continuous Key space under heavy pressure can cause hot spots, causing storage_server_write_queue_size or even process_behind. However, such hot spots may be known to the upper layer services but cannot be avoided. I wish there was an API that partitioned the shards ahead of time so that the write load was evenly distributed across multiple storageservers. In this way, the upper business can actively divide the Keys space to reduce hot spots.

Start by reviewing FoundationDB's existing range and shard-management APIs, then trace how storage servers handle write queues and shard placement. Define the API boundary and behavior needed to partition a key range ahead of time, and identify tests that can demonstrate more even distribution without introducing hot spots.

Written by the indexing model from the issue text.