Test endpoint using the set SQL_LOGIN_TIMEOUT value

With this change when the entered endpoint is unreachable Tableau will
warn the user during connection setup instead of proceeding with an
incorrect endpoint and failing during query time. The timeout value set
by Tableau is used when testing the endpoint.
Solid plan :+1: 

One small comment: "endpoint is not UP" is less accurate than "Cannot establish connection with endpoint". We would not know whether the endpoint is actually online or whether we simply cannot route to it.