Run npm in a separate worker process

This is an experiment. It establishes a nice security wall between npm and etherpad helps keep memory usage of the main process low.

I'm not sure whether it should keep the same process running the whole time (it's allocated around 300-500Mb on my machine, if I read the stats correctly), on the other hand, creating a new child process for every plugin search is probably too costly, too.

Maybe we can just throw in a timeout of say 5mins and after that the npm worker is killed.

This addresses #1975, I'm not sure it will solve #1992