Logging disappears when worker processes in use

[jeromekelleher]

When we run vcf2zarr encode -p0 -vv we get lots of log output, but with p1 and greater this disappears.

Related to #296

[jeromekelleher]

Note that this seems to be due to us not managing logging from the subprocesses in any meaningful way. It seemed to work with some versions of Python, which must be why I missed it.

[jeromekelleher]

So there's not much to be done here in the short term I think, unfortunately there's no easy workaround. The best thing to do would be to setup a custom logging handler using queues etc as demo'd in the Python docs but this is a significant engineering task.

In the short term we need to document how to work around this when having memory issues.