Configurable worker ID source

Currently, it's Random. Normal flakes use the MAC address.

We changed it to a random for safety reason in the Pleroma ecosystem and because breaking this unique safety was okay (we don't run clusters). However, some people do run huge clusters, care less about leaking some information and need guarantees which they could get with the MAC source.

Probably, the default worker ID source should be Random.

Suggested by @href.