Change --max-worker-restart option to a sensible value

[nicoddemus]

The default value allows for infinite restarts, but this is a serious problem for suites which are during collection because the test run never finishes.

We should change the default to a sensible number, @RonnyPfannschmidt suggested 4 times the number of nodes as default. A value of 0 would mean the previous behavior.

[timj]

I strongly endorse this. We use C++ python modules and sometimes there is a SEGV and it took us a while to understand why we ended up with locked up machines forking processes until the machine begged for mercy. It's highly unlikely that in our case a restart would ever be the right thing to do.

[blueyed]

@timj
Care to create a PR for this?