YARN-5904. Reduce the number of default server threads for AMRMProxyService.

(cherry picked from commit 140b9939da71ec51c178162501740a429b344cac)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-api/src/main/java/org/apache/hadoop/yarn/conf/YarnConfiguration.java

@@ -1706,7 +1706,7 @@ public class YarnConfiguration extends Configuration {
 
   public static final String AMRM_PROXY_CLIENT_THREAD_COUNT = NM_PREFIX
       + "amrmproxy.client.thread-count";
-  public static final int DEFAULT_AMRM_PROXY_CLIENT_THREAD_COUNT = 25;
+  public static final int DEFAULT_AMRM_PROXY_CLIENT_THREAD_COUNT = 3;
 
   public static final String AMRM_PROXY_INTERCEPTOR_CLASS_PIPELINE =
       NM_PREFIX + "amrmproxy.interceptor-class.pipeline";