Unique names for bulk processor scheduler threads (#69432)

Today every `BulkProcessor` creates two scheduler threads, both called
`[node-name][scheduler][T#1]`, which is also the name of the main
scheduler thread for the node. The duplicated thread names make it
harder to interpret a thread dump.

This commit makes the names of these threads distinct.

Closes #68470

qa/multi-cluster-search/src/test/java/org/elasticsearch/search/CCSDuelIT.java

@@ -193,7 +193,7 @@ public class CCSDuelIT extends ESRestTestCase {
                 public void afterBulk(long executionId, BulkRequest request, Throwable failure) {
                     throw new AssertionError("Failed to execute bulk", failure);
                 }
-            }).build();
+            }, "CCSDuelIT").build();
 
         int numQuestions = randomIntBetween(50, 100);
         for (int i = 0; i < numQuestions; i++) {