GEODE-8696: Remove redundant syncs to prevent deadlock (#5894)

FederatingManager now uses a ReentrantLock instead of synchronization.
diff --git a/geode-core/src/main/java/org/apache/geode/management/internal/FederatingManager.java b/geode-core/src/main/java/org/apache/geode/management/internal/FederatingManager.java
index 3ba7c21..afc51d4 100755
--- a/geode-core/src/main/java/org/apache/geode/management/internal/FederatingManager.java
+++ b/geode-core/src/main/java/org/apache/geode/management/internal/FederatingManager.java
@@ -112,7 +112,7 @@
    * Management exception has to be handled by the caller.
    */
   @Override
-  public synchronized void startManager() {
+  public void startManager() {
     try {
       lifecycleLock.lock();
       try {
@@ -162,7 +162,7 @@
   }
 
   @Override
-  public synchronized void stopManager() {
+  public void stopManager() {
     lifecycleLock.lock();
     try {
       // remove hidden management regions and federatedMBeans
@@ -352,7 +352,7 @@
     }
   }
 
-  private synchronized void executeTask(Runnable task) {
+  private void executeTask(Runnable task) {
     try {
       executorService.get().execute(task);
     } catch (RejectedExecutionException ignored) {