We cannot control anymore the maximum number of URLs using the API
because now we are using check-in, that has no such limit.
We could theoretically clamp the number of URLs to measure after
the call to check-in, and still honour the setting.
Yet, the right thing to do seems to introduce a max runtime variable
because that is what desktop and mobile do.
Thus, introduce code that warns the user about the change in the
settings, should they have set the URL limit to nonzero.
We are going to do a best effort conversion from the URL limit to
the maximum runtime for the rest of 2021.
Since then, we will silently ignore the URL limit.
This work is part of https://github.com/ooni/probe/issues/1299.
