HBASE-26775 - add synchronized modifier to the toString() method of ProcedureEvent.java (#4681)

Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit 74726617b8418f9cd0b1f3ae209cfc42adbf26cd)

hbase-procedure/src/main/java/org/apache/hadoop/hbase/procedure2/ProcedureEvent.java

@@ -126,7 +126,7 @@ public class ProcedureEvent<T> {
   }
 
   @Override
-  public String toString() {
+  public synchronized String toString() {
     return getClass().getSimpleName() + " for " + object + ", ready=" + isReady() + ", "
       + suspendedProcedures;
   }