Support Statement.closeOnCompletion.

Check a statement after all its dependent result set are closed.

Extract TarantoolStatement as tarantool specific extension interface to
be used for internal purposes (incompatible vendor API).

Closes: tarantool#180

Author: nicktorwald

src/test/java/org/tarantool/TarantoolConnectionSQLOpsIT.java

@@ -22,7 +22,9 @@ public void setup() {
 
     @AfterEach
     public void tearDown() {
-        connection.close();
+        if (connection != null) {
+            connection.close();
+        }
     }
 
     @Override

src/test/java/org/tarantool/jdbc/JdbcConnectionTimeoutIT.java

@@ -71,7 +71,9 @@ protected void completeSql(TarantoolOp<?> operation, TarantoolPacket pack) {
 
     @AfterEach
     void tearDown() throws SQLException {
-        connection.close();
+        if (connection != null) {
+            connection.close();
+        }
     }
 
     @Test