Make JDBC identifier mapping volatile

(cherry picked from commit b7dcb6c)

Author: suxiaogang223

fe/fe-core/src/main/java/org/apache/doris/datasource/jdbc/JdbcExternalCatalog.java

@@ -77,7 +77,7 @@ public class JdbcExternalCatalog extends ExternalCatalog {
     // Must add "transient" for Gson to ignore this field,
     // or Gson will throw exception with HikariCP
     private transient JdbcClient jdbcClient;
-    private IdentifierMapping identifierMapping;
+    private volatile IdentifierMapping identifierMapping;
     private ExternalFunctionRules functionRules;
 
     public JdbcExternalCatalog(long catalogId, String name, String resource, Map<String, String> props,