DERBY-3888: ALTER TABLE ... ADD COLUMN cannot add identity columns

Enable support for adding identity columns with ALTER TABLE. It is
only enabled if identity columns are backed by sequences. That is, the
database format has to be 10.11 or higher.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1626141 13f79535-47bb-0310-9956-ffa450edef68