Soft delete feature for comment [NO MERGE]

@ajs6f I think the resource status flag that @awoods is very much an application concern and I don't understand why fcrepo4 needs special baked-in-support for it.

This patch is essentially about tombstoning for deleted objects, and if we're going to bother with that, we might as well satisfy the soft-delete use case (which I understand to mean, DELETE doesn't remove data). In fact, true tombstones (without soft-delete at all) would be more useful (IMO), and is all that's necessary to satisfy LDP:

```
5.2.3.11 LDP servers that allow member creation via POST should not re-use URIs.

6.1.2 LDP servers should not re-use URIs, regardless of the mechanism by which members are created (POST, PUT, etc.). Certain specific cases exist where a LDPC server might delete a resource and then later re-use the URI when it identifies the same resource, but only when consistent with Web architecture. While it is difficult to provide absolute implementation guarantees of non-reuse in all failure scenarios, re-using URIs creates ambiguities for clients that are best avoided.
```