Garbage collection write amplification

[sanebay]

Because GC is a heavy operation and can cause unnecessary write amplification, ti should do following.

1. Group shards of similar TTL or buckets across customers to be together. Even if TTL is some arbitrary number, we can group them, so that we dont have to move data and we can release chunk with a quick metadata update.
2. DM can send some hints as part of create shard and chunk selector tries its best to group similar shards in same chunk.
3. Keep track of how many times a shard has been moved in the shard header. This needs to be exported via metrics.