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.
Few more nits:
1. Rename to `canConnect`. This expresses what the values of the bool will actually *mean* a bit better :)
2. Give timeout a default value, as in, `size_t timeout = 5`
3. Reverse the parameter order. D has this lovely feature where you can call any normal function like it is a method, for example:
`void foo(string bar);`

Can be invoked in these two ways:
`foo("Hello")`
`"Hello".foo`

In this case, `endpoint.canConnect(5)` seems like a reasonable way to call it.