[fix](fe) modify tablet cooldownConfLock to reduce memory (#59356)

### What problem does this PR solve?

the usage of `cooldownConfLock` is simple, does not need
ReentrantReadWriteLock because it cost too much memory, especially when
a cluster has many tablets.

before:
<img width="1280" height="575" alt="1"
src="https://github.com/user-attachments/assets/df626e1e-84ef-4078-8da7-d857ac960678"
/>

after:
<img width="1280" height="510" alt="2"
src="https://github.com/user-attachments/assets/3fee7407-0743-4d07-9462-a9f0e101765f"
/>

Author: mymeiyi

fe/fe-core/src/main/java/org/apache/doris/catalog/Tablet.java

@@ -25,7 +25,6 @@
 import org.apache.doris.common.FeConstants;
 import org.apache.doris.common.Pair;
 import org.apache.doris.common.UserException;
-import org.apache.doris.common.lock.MonitoredReentrantReadWriteLock;
 import org.apache.doris.resource.Tag;
 import org.apache.doris.system.Backend;
 import org.apache.doris.system.SystemInfoService;
@@ -123,7 +122,7 @@ public TabletHealth() {
     private long cooldownReplicaId = -1;
     @SerializedName(value = "ctm", alternate = {"cooldownTerm"})
     private long cooldownTerm = -1;
-    private MonitoredReentrantReadWriteLock cooldownConfLock = new MonitoredReentrantReadWriteLock();
+    private final Object cooldownConfLock = new Object();
 
     // last time that the tablet checker checks this tablet.
     // no need to persist
@@ -184,22 +183,19 @@ public boolean isConsistent() {
     }
 
     public void setCooldownConf(long cooldownReplicaId, long cooldownTerm) {
-        cooldownConfLock.writeLock().lock();
-        this.cooldownReplicaId = cooldownReplicaId;
-        this.cooldownTerm = cooldownTerm;
-        cooldownConfLock.writeLock().unlock();
+        synchronized (cooldownConfLock) {
+            this.cooldownReplicaId = cooldownReplicaId;
+            this.cooldownTerm = cooldownTerm;
+        }
     }
 
     public long getCooldownReplicaId() {
         return cooldownReplicaId;
     }
 
     public Pair<Long, Long> getCooldownConf() {
-        cooldownConfLock.readLock().lock();
-        try {
+        synchronized (cooldownConfLock) {
             return Pair.of(cooldownReplicaId, cooldownTerm);
-        } finally {
-            cooldownConfLock.readLock().unlock();
         }
     }