Ensure swift.source is set for DLO/SLO requests

SLO was setting it sometimes (the PUT path), but not others (the GET
path), and DLO just wasn't doing it at all. One could already
distinguish DLO/SLO internal requests by the user agent, but we should
set swift.source too.

Change-Id: I5d5b7dd49bd1522a9c830d0abd21fff92ae79a39
Merge "Ensure swift.source is set for DLO/SLO requests"