[Libguestfs] [PATCH libnbd discussion only 0/5] api: Implement concurrent writer.

This works, but there's no time saving and I'm still investigating whether it does what I think it does. Nevertheless I thought I would post it because it (probably) implements the idea I had last night outlined in: https://www.redhat.com/archives/libguestfs/2019-June/msg00010.html The meat of the change is patch 4. Patch 5 is an example which I would probably fold into patch 4 for submission. The others are non-controversial documentation and refactorings to prepare for the change. Rich.