This enables sharded DNS publishing on a per-TLD basis. Instead of a TLD-wide lock, the sharded scheme locks each update on the shard number, allowing parallel writes to DNS.
We allow N (the number of shards) to be 0 or 1 for no sharding, and N > 1 for an N-way sharding scheme. Unless explicitly set, all TLDs default to a numShards of 0, so we don't have to reload all registry objects explicitly.
WARNING: This will change the lock name upon deployment for the PublishDnsAction from "<TLD> Dns Updates" to "<TLD> Dns Updates shard 0". This may cause concurrency issues if the underlying DNSWriter is not parallel-write tolerant (currently all production usages are ZonemanWriter, which is parallel-tolerant, so no issues are expected).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187525655
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