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)
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html