ENH: improved handling of regIOobject deletion (#1276)

- remove registration and ownership before deleting a regIOobject
  from within objectRegistry to avoid possible recursion.

- regIOobject destructor now removes any registered object from
  registry regardless if ownedByRegistry or not. It also removes
  always removes the ownership flag to avoid possible recursion.

- the regIOobject::checkOut() now unconditionally clears file watches.
  These will only be there if the object is registered (a no-op for an
  unregistered object), but this additional safety is needed to manage
  case where the registration has been modified elsewhere (eg, by the
  objectRegistry).