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.
In D it is also usually a bad sign if you're resorting to pointers (as there are safer/faster abstractions readily available). In this case, I think the thing to do is just add a normal data member, `size_t connectionTimeout;`. The purpose of this class is to simply hold/group all of this stuff.