[fix](ForkJoinPool) we should not new a thread pool every call (apach…

…e#44891)

### What problem does this PR solve?

come from: apache#34988

Problem:
Use JDK11 and call `show proc "/cluster_health/tablet_health"`
frequently, make large number of ForkJoinPool thread leak

fe/fe-core/src/main/java/org/apache/doris/common/proc/TabletHealthProcDir.java

@@ -67,7 +67,7 @@ public class TabletHealthProcDir implements ProcDirInterface {
 
     private Env env;
 
-    private ForkJoinPool taskPool = new ForkJoinPool();
+    private static ForkJoinPool taskPool = new ForkJoinPool();
 
     public TabletHealthProcDir(Env env) {
         Preconditions.checkNotNull(env);