read soft_delete from provider before acting

[philbrookes]

In order to avoid race conditions on soft_delete which could lead to an NXDomain response we should first write the soft_delete label to the DNS Provider as a way of signalling and intention to delete.

The delete can then be performed once read back from the provider and assuring that unlabelled endpoints still remain.

Note that this requires a change to the TXT registry to function: #346