This still retains the DnsWriter interface itself for better integration
with Dagger and to preserve the option of having a DNS writer that does
not have this requirement (e.g. because it is idempotent).
This also makes the commit check thread-safe, which is a nice-to-have.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168451114
