Set req.hash_ignore_busy = true for PURGE requests

PURGE requests should never wait for busy objects to finish a fetch.
I suspect this may be the reason why some PURGE requests take a long
time.

Change-Id: I77d54b5826237228e0d6847743d3d1c0770f29c8
Merge "Set req.hash_ignore_busy = true for PURGE requests" into production