Ordinarily, an ENOENT in _finalize_durable should mean something's gone
off the rails -- we expected to be able to mark data durable, but
couldn't!
If there are concurrent writers, though, it might actually be OK:
Client A writes .data
Client B writes .data
Client B finalizes .data *and cleans up on-disk files*
Client A tries to finalize but .data is gone
Previously, the above would cause the object server to 500, and if
enough of them did this, the client may see a 503. Now, call it good so
clients get 201s.
Change-Id: I4e322a7be23870a62aaa6acee8435598a056c544
Closes-Bug: #1719860
69b8165cd8