* Fix deadlock and DELETE/UPDATE unindexed datasets

PBENCH-1328

A bug revealed another bug:

1. `POST`/`DELETE` `/datasets/{id}` fails when the dataset isn't indexed; with
the ability to disable server indexing entirely and to remove dataset indexed
data, this is undesirable. As long as the dataset doesn't have a `WORKING` ops
status, we should be able to update or delete.
2. When the operation fails, the status is already set to `WORKING`, and this
was not corrected on exit, leaving the dataset locked perpetually.