Added logging of x-delete-at values

Added logging of the x-delete-at value for object PUTs and POSTs and
made the additional log field generic for other similar use cases.

Logging the x-delete-at is useful because otherwise you might never
know why an object "disappeared" just by looking at saved proxy logs.
You'd see a successful PUT of the object followed some time by a GET
that 404s. If you still have the other logs, you could track down
what happened; but, at least with our clusters, we keep proxy logs
forever but only keep the other logs for a short while.

Change-Id: Ida559abcf8dc5666e2a7eea250a67d1c4b28b1be
Merge "Added logging of x-delete-at values"