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.