Avoid issues on concurrent writes with SQL Server

Microsoft SQL Server is currently used with SNAPSHOT isolation level.

The problem with this isolation level is that it prevents concurrent writes. They result in:

Snapshot isolation transaction aborted due to update conflict. You cannot use snapshot isolation to access table 'dbo.hierarchy' directly or indirectly in database 'nuxeo' to update, delete, or insert the row that has been modified or deleted by another transaction. Retry the transaction or change the isolation level for the update/delete statement.

We should switch to the READ COMMITTED isolation level, which still using row-based versioning if the database is properly configured with ALTER DATABASE nuxeo SET READ_COMMITTED_SNAPSHOT ON (http://doc.nuxeo.com/x/EgI7).