Default to --workers=1

[smola]

From @dennwc (more):

• After 3-5 min of activity, Borges consumend all the memory available on the laptop and after 20 minutes of complete OS freeze it finally got an OOM kill from the kernel.
  • Reduced the number of workers from 8 to 4 and started to monitor the memory closely.
  • Memory consumtion seemed to be OK for some time, but when I stopped looking at it, the laptop froze again and Borges got another OOM kill.
  • Reduced the number of workers to 1. Seems stable now.

We are using --workers=1 ourselves in our deployments, partly because of this reason. So it would be good to default to it and spare the trouble to users.

In readme, instead of using --workers=20 as an example for how they can be changed, we could use --workers=4.