Enable switching Rx versions for benchmarks.

[danielcweber]

This PR makes the referenced Rx-version switchable. It works by force-overriding the used Rx-version (if taken from nuget). It can be combined with the standalone solution from #730, the solution should then have configurations that match the project configurations (current, Rx3.1.1, etc), so the config can easily be switched in the VS dropdown.

[akarnokd]

I'm not sure how this would work with #730 as it is a separate solution and has no access to CURRENT.

Besides, I found it much easier to keep the NuGet manager open and switch the Rx library with one dropdown and one update click. This PR requires clicking on the releases dropdown, clicking on the configuration manager, clicking on the configuration dropdown, clicking on it again because the first click just selects the cell, click on the setting, close the window and finally clean the solution.

Regardless, I don't see a reason to keep both of them around and copy benchmarks between the two.

[danielcweber]

I'm not sure how this would work with #730 as it is a separate solution and has no access to CURRENT.

Unless you reference the project in the separate solution.

Besides, I found it much easier to keep the NuGet manager open and switch the Rx library with one dropdown and one update click.

That's what I'm saying, the hybrid of #730 and #731 can have both, access to the current sources and easier access to the configuration.

I'm good with either.