Set the number of map shards to 20

This change is motivated by the sandbox run where we saw the backend instances overwhelmed by the 100 default shards to the point where they couldn't even answer a simple status request.

Production has 50 backend instances, so 20 will leave a lot of spare for other tasks.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162357857

java/google/registry/backup/DeleteOldCommitLogsAction.java

@@ -67,6 +67,7 @@ import org.joda.time.Duration;
 )
 public final class DeleteOldCommitLogsAction implements Runnable {
 
+  private static final int NUM_MAP_SHARDS = 20;
   private static final int NUM_REDUCE_SHARDS = 10;
   private static final FormattingLogger logger = getLoggerForCallerClass();
 
@@ -86,6 +87,7 @@ public final class DeleteOldCommitLogsAction implements Runnable {
     response.sendJavaScriptRedirect(createJobPath(mrRunner
           .setJobName("Delete old commit logs")
           .setModuleName("backend")
+          .setDefaultMapShards(NUM_MAP_SHARDS)
           .setDefaultReduceShards(NUM_REDUCE_SHARDS)
           .runMapreduce(
               new DeleteOldCommitLogsMapper(),