Add X-Backend-Timestamp on more object server responses

It's particularly interesting on writes (PUT, POST, DELETE) where the
current on-disk timestamp would prevent the object server from serving
the incoming request and returns 409 Conflict.

The FakeConn has also been updated to respond in kind for 409's on
expect and all responses generaly just cause it's good to keep fakes in
line with the reals - not that I expected any existing tests to break
because of the new headers.

Change-Id: Iac6fbd2f872a9521bb2db84a333365b69f54fb6c
Merge "Add X-Backend-Timestamp on more object server responses"