The libnm API fir checkpoints was only introduced with 1.11. It
is not yet stable, so there is still time to adjust it. Note that
this changes API/ABI of the development branch.

Changes:

- we only add async variants of the checkpoint functions. I believe
  that synchronous D-Bus methods are fundamentally flawed, because
  they mess up the ordering of events.
  Rename the async functions by removing the "_async" suffix. This
  matches glib style, for which the async form is also not specially
  marked.

- for function that refere to a particular checkpoint (rollback and
  destroy), accept the D-Bus path as string, instead of an NMCheckpoint
  instance. This form is more flexible, because it allows to use
  the function without having a NMCheckpoint instance at hand. On the
  other hand, if one has a NMCheckpoint instance, he can trivially
  obtain the path to make the call.