Merge branch 'fix-mp-win-3.0' into 'master'

Fix multiprocessing on Windows

*Related to:*  #184 

I'm still running into #179 with this fix but all processes start now at least.

BTW the current design is pretty bad regarding memory footprint and bootstrap time as you have to pickle the manager object for *every* subprocess. So this does not only fix multiprocessing on Windows. It improves the multiprocessing design on other platforms, too.

See merge request !97