This saves a few bytes in the response size and make it easy
for memcached proxies to distinguish key fetches that are part
of check-and-set cycles from those that are not.
MediumSpecificBagOStuff now requires PASS_BY_REF to fetch CAS
tokens. BagOStuff::merge() and WinCacheBagOStuff::doCas() are
the only callers that need this mode.
Bug: T257003
Change-Id: If91963f58adc4cda94f6d634ee0252a479a0fc5e
