This commit removes the namespace kv worker and related code, now that
we talk over gRPC to the server, and not directly to the DB, we should
not need this anymore.
This commit rewrites a bunch of the code to always use *Machine instead
of a mix of both, and a mix of tailcfg.Node and Machine.
Now we use *Machine, and if tailcfg.Node is needed, it is converted just
before needed.
tests for that feature.
Other fixes: clean up a few typos in comments. Fix a bug that caused the
tests to run four times each. Be more consistent in the use of log
rather than fmt to print errors and notices.
