Restores promise that it's safe to re-destroy a scope.

https://github.com/square/mortar/issues/90
Adds MortarScope#isDestroyed

https://github.com/square/mortar/issues/90
Better tests of idempotence of destroying, per review.
Merge pull request #91 from square/ray/double-destroy

Fixes rough edges of new parent-based destroy scheme.