Default<> is too easy to use incorrectly

[marcus-cellavision]

I think the Default<> template is hard to use and easily leads to mistakes, or at least multiple possibly unintended default executor instances (and thus more threads than intended).

In particular the static instances are not necessarily unique, globally, e.g. when used in different DLLs on Windows. I don't think this is the intention (?), however, the APIs make it all too easy not to provide an executor (and defaulting to Default<Executor>).