Decrease transaction_clean_interval_second config (#2673)

fe/src/main/java/org/apache/doris/common/Config.java

@@ -119,8 +119,9 @@ public class Config extends ConfigBase {
     @ConfField public static int label_clean_interval_second = 4 * 3600; // 4 hours
     /*
      * the transaction will be cleaned after transaction_clean_interval_second seconds if the transaction is visible or aborted
+     * we should make this interval as short as possible and each clean cycle as soon as possible
      */
-    @ConfField public static int transaction_clean_interval_second = 300; // 5 min
+    @ConfField public static int transaction_clean_interval_second = 30;
 
     // Configurations for meta data durability
     /*

fe/src/main/java/org/apache/doris/transaction/GlobalTransactionMgr.java

@@ -821,6 +821,8 @@ public class GlobalTransactionMgr implements Writable {
      * The txn cleaner will run at a fixed interval and try to delete expired and timeout txns:
      * expired: txn is in VISIBLE or ABORTED, and is expired.
      * timeout: txn is in PREPARE, but timeout
+     * Todo(kks): currently remove transaction performance is bad, if we want to support
+     *  super-high concurrent transaction, we should improve this method
      */
     public void removeExpiredAndTimeoutTxns() {
         long currentMillis = System.currentTimeMillis();