add conf entry jdbc.storage_engine to change storage engine

hugegraph-mysql/src/main/java/com/baidu/hugegraph/backend/store/mysql/MysqlOptions.java

@@ -97,4 +97,12 @@ public static synchronized MysqlOptions instance() {
                     disallowEmpty(),
                     "disable"
             );
+
+    public static final ConfigOption<String> STORAGE_ENGINE =
+            new ConfigOption<>(
+                   "jdbc.storage_engine",
+                   "The storage engine of backend store database, like InnoDB/MyISAM/RocksDB for MySQL.",
+                    disallowEmpty(),
+                    "InnoDB"
+            );
 }

hugegraph-mysql/src/main/java/com/baidu/hugegraph/backend/store/mysql/MysqlSessions.java

@@ -235,6 +235,10 @@ public Session() {
             }
         }
 
+        public HugeConfig config() {
+            return MysqlSessions.this.config();
+        }
+
         public void open() throws SQLException {
             if (this.conn != null && !this.conn.isClosed()) {
                 return;

hugegraph-mysql/src/main/java/com/baidu/hugegraph/backend/store/mysql/MysqlTable.java

@@ -104,7 +104,7 @@ protected void createTable(Session session, TableDefine tableDefine) {
             }
         }
         sql.append("))");
-        sql.append(this.engine());
+        sql.append(this.engine(session));
         sql.append(";");
 
         LOG.debug("Create table: {}", sql);
@@ -116,8 +116,9 @@ protected void createTable(Session session, TableDefine tableDefine) {
         }
     }
 
-    protected String engine() {
-        return " ENGINE=InnoDB";
+    protected String engine(Session session) {
+        String engine = session.config().get(MysqlOptions.STORAGE_ENGINE);
+        return " ENGINE=" + engine;
     }
 
     protected void dropTable(Session session) {

hugegraph-postgresql/src/main/java/com/baidu/hugegraph/backend/store/postgresql/PostgresqlTable.java

@@ -26,6 +26,7 @@
 
 import com.baidu.hugegraph.backend.store.mysql.MysqlBackendEntry;
 import com.baidu.hugegraph.backend.store.mysql.MysqlTable;
+import com.baidu.hugegraph.backend.store.mysql.MysqlSessions.Session;
 import com.baidu.hugegraph.type.define.HugeKeys;
 
 public abstract class PostgresqlTable extends MysqlTable {
@@ -46,7 +47,7 @@ protected String buildTruncateTemplate() {
     }
 
     @Override
-    protected String engine() {
+    protected String engine(Session session) {
         return Strings.EMPTY;
     }