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.