Used when the connection is about to be closed. Typically this will involve flushing data
from caches, but performance is a consideration, so it should only perform what might be
necessary for closing the file. If anything must happen when a file is closed, it must be
implemented in the Node::close function, not here; a call to sync with this mode is not
guaranteed and not implementing/supporting it should have no effect on correctness. If
Node::close needs to flush data in an async context, it has to spawn a task. Supporting
this mode means that in most cases there’s no need to spawn a task because there should be
nothing that needs to be flushed (but it must check). This will only be called if the
connection has write permissions; a connection that only has read permissions should not
have made any changes that need flushing.