Make default worker_processes 0 #404
Description
Currently we have a default of 1 worker process, which means that we create a single background process to do encoding. This is a bit pointless really, resulting in some extra memory usage for the tskit backend (as we pickle/copy the tree sequence into the worker) and also hides log output by default (#302)
To implement workers=0 we use the core.SynchronousExecutor which creates Future objects explicitly, which is not recommended in the Python docs. However, we don't seem to have hit any issues with this, so I think the benefits outweigh the risk here and we can set default worker processse to 0 by default.