This allows static symlinks to be synced before their target. Dynamic
symlinks could already be synced even if target object has not been
synced, but static links previously required that target object existed
before it can be PUT. Now, have container_sync middleware plumb in an
override like it does for SLO.
Change-Id: I3bfc62b77b247003adcee6bd4d374168bfd6707d