Introduce Shard IDs to logically group nodes in cluster mode.
1. Added a new "shard_id" field to "cluster nodes" output and nodes.conf after "hostname"
2. Added a new PING extension to propagate "shard_id"
3. Handled upgrade from pre-7.2 releases automatically
4. Refactored PING extension assembling/parsing logic
Behavior of Shard IDs:
Replicas will always follow the shards of their reported primaries. If a primary updates its shard ID, the replica will follow. (This need not follow for cluster v2) This is not an expected use case.
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