fix schema migration hooks (partial)
SQL Server requires some additional steps to alter columns
(index drop and restore, etc...) and this in turn requires
passing a connection on the DB to find out the indexes.
Also partially fix an issue where the null_allowed value was
not correctly computed. Still some problems in there
(see https://www.logilab.net/cwo/ticket/1109558)
# copyright %(year)s %(author)s, all rights reserved.
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"""%(distname)s schema"""