Clarify PUT location with chunked upload

When uploading a blob in chunks, the spec says:

To upload a chunk, issue a PATCH request to a URL path in the following format, and with the following headers and body:

URL path: <location>

[...]

The <location> refers to the URL obtained from the preceding POST request.

This implies to me that <location> can change with each successive POST, so there are potentially
n + 1 locations in play for n chunks (one for the initial POST and one for each PATCH).

The final PUT request is documented as follows:

To close the session, issue a PUT request to a url in the following format, and with the following headers (and optional body, depending on whether or not the final chunk was uploaded already via a PATCH request):

<location>?digest=<digest>

This doesn't make it clear which <location> should be used. Should it be the location returned by the most recent PATCH request, or the <location> returned by the original POST?

Start with the chunked-upload section quoted in issue #415 and trace how the location is described for the initial POST, successive PATCH requests, and final PUT. Clarify which location the final PUT must use, then update the specification so the intended behavior is unambiguous.