added support of LOB download

OCI8Statement::bindParam() was forcing a LOB upload when passing a
parameter of a LOB type. Also, this forced upload is out of concern for
OCI8Statement::bindParam().

So I removed it to support LOB download. Now call of
oci_new_descriptor() to create a LOB is the responsibility of the
controller. To help this call I added OCI8Connection::getDBH() to be
able to get the resource to the connection.

File download example:
```
$connection = new \Doctrine\DBAL\Connection(...);
$sql = 'BEGIN APP.PKG_NAME.GET_FILE(:id,:data); END;';
$stmt = $connection->prepare($sql);
$stmt->bindValue('id', $id);

$blob = oci_new_descriptor($connection->getWrappedConnection()->getDBH(), OCI_DTYPE_LOB);
$stmt->bindParam('data', $blob, PDO::PARAM_LOB);

$stmt->execute();
$data = $blob->load();
```

File upload example:
```
$connection = new \Doctrine\DBAL\Connection(...);
$sql = 'BEGIN APP.PKG_NAME.PUT_FILE(:id,:bData); END;';
$stmt = $connection->prepare($sql);
$stmt->bindValue('id', $id);

$blob = oci_new_descriptor($connection->getWrappedConnection()->getDBH(), OCI_DTYPE_LOB);
$blob->writeTemporary($data, OCI_TEMP_BLOB);
$stmt->bindParam('data', $blob, PDO::PARAM_LOB);

$stmt->execute();
```