Fix Azure Disconnect problem.

I'm poor at English.
If you can't understand what I mean, please let me know.

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Azure SQL close the client connection every 5min.
FreeTDS can't diagnose it.

This is a specification of Azure SQL.
http://blogs.msdn.com/b/bartr/archive/2010/06/18/sql-azure-connection-retry.aspx

Adapter needs execute SQL.
But, this method is too slow, because client waits timeout.
That's way this pull request measure the time (5min) and execute SQL.

Thanks, does this execute need to be here?