Improve nodestore benchmarking:
* Use more succinct while loops on NodeFactory.
* Better formatting of multiple test results.
* Updated benchmarks.
* Use simpler and faster RNG to generate test data.
Fix Overlay stop on exit:
The stop sequence for Overlay had a race condition where autoconnect could
be called after close_all, resulting in a hang on exit. This resolves the
problem by putting the close and timer operations on a strand:
* Rename some Overlay members
* Put close on strand and tidy up members
* Use completion handler instead of coroutine for timer