Verify deletes in backup restore test

Using the new function to detect when tombstones are really, really gone
so the test is more reliable.
Also, a bit of refactoring of the delete verification and wait_until
functions to make it cleaner.
Fix wait_until_registered

It worked only because it's only used with riak_core_ring_manager now,
but it wouldn't really work with any other name. h/t Sean's good eye
(the left one).
Merge pull request #320 from basho/eas-verify-deletes-in-backup-restore

Verify deletes in backup restore test