Extract multiple commit prevention in DNS writers into a base class

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.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168451114

java/google/registry/dns/writer/VoidDnsWriter.java

@@ -14,8 +14,6 @@
 
 package google.registry.dns.writer;
 
-import static com.google.common.base.Preconditions.checkState;
-
 import com.google.common.base.Joiner;
 import java.util.HashSet;
 import java.util.Set;
@@ -27,7 +25,7 @@ import javax.inject.Inject;
  *
  * <p>All this class does is write its displeasure to the logs.
  */
-public final class VoidDnsWriter implements DnsWriter {
+public final class VoidDnsWriter extends BaseDnsWriter {
 
   /**
    * The name of the pricing engine, as used in {@code Registry.dnsWriter}. Remember to change
@@ -37,8 +35,6 @@ public final class VoidDnsWriter implements DnsWriter {
 
   private static final Logger logger = Logger.getLogger(VoidDnsWriter.class.getName());
 
-  private boolean committed = false;
-
   private final Set<String> names = new HashSet<>();
 
   @Inject
@@ -55,10 +51,7 @@ public final class VoidDnsWriter implements DnsWriter {
   }
 
   @Override
-  public void commit() {
-    checkState(!committed, "commit() has already been called");
-    committed = true;
-
+  protected void commitUnchecked() {
     logger.warning("Ignoring DNS zone updates! No DnsWriterFactory implementation specified!\n"
         + Joiner.on('\n').join(names));
   }