`Node#close` is pretty hard to rely on today:
 - it might swallow exceptions
 - it waits for 10 seconds for threads to terminate but doesn't signal anything
   if threads are still not terminated after 10 seconds

This commit makes `IOException`s propagated and splits `Node#close` into
`Node#close` and `Node#awaitClose` so that the decision what to do if a node
takes too long to close can be done on top of `Node#close`.

It also adds synchronization to lifecycle transitions to make them atomic. I
don't think it is a source of problems today, but it makes things easier to
reason about.