Improve list SQL Server table indexes

This PR improves listing table indexes in SQL Server. Currently table indexes are retrieved in the platform via a stored procedure <code>sp_helpindex</code> which causes problems with some SQL Server drivers like ODBC that cannot retrieve information from stored procedure calls. Table indexes are retrieved via native system tables now, which seems cleaner and is more flexible concerning which information to retrieve.
Additionally code duplication in <code>SQLServerSchemaManager::_getPortableTableIndexesList</code> is removed and simplified by this PR.