Dist object move enhancements

[Dan Bonachea]

Two related enhancement proposals arising from spec PR 97:

1. dist_object move assignment and move construction both currently require library initialization and the master persona, even for inactive source objects (in both spec and impl). However this seems unnecessary and I suggest we conditionally relax both preconditions when !other.is_active().

2. dist_object<T> move construction currently requires MoveAssignable T, although this is an artifact of our current implementation (using move assign to implement move construction) and not fundamentally necessary (nor was it intended to restrict usage in this way). We should relax that constraint in both spec and implementation.