NMCheckpointManager was added and is not ref-countable, because it
is not needed.

I still often like for such objects (that are not ref-countable),
that their destroy function is called "unref". Both for consistency,
and also if we would later add ref-counting to the object.

However, NMCheckpointManager keeps a pointer to NMManager. So, when
NMManager gets destroyed, it *MUST* destroy the NMCheckpointManager.
It cannot accept that the checkpoint manager outlives NMManager,
but the "unref" name suggests that somebody else might have still
a reference to this object keeping it alive. That is not the case.

Rename so that this is clear.

I would name it nm_checkpoint_manager_destroy(), but "destroy" already
has a meaning for NMCheckpoint instances, so use "free".