Blanking preferred kept route-by-key reads from dialing a flagged owner first,
but withFilerClientFailover always re-adds the current filer, so when the owner
is the gateway's current filer it stayed in the candidate list and got dialed
anyway. Treat a recently-unreachable filer as unhealthy in the health partition
so it is deferred to the last-resort tail instead of tried before healthy
replicas; preferred is still tried first, and a live owner is unaffected.
Chris Lu
ef1aa4f936