* wdclient: prune filers dropped from master discovery
Filer discovery only appended new addresses; it never removed ones that
disappeared from the master snapshot. After a K8s filer pod rolled to a
new IP the old address lingered in filerAddresses and got retried again
every resetTimeout window, stalling S3 uploads on i/o timeouts.
Treat the master snapshot as authoritative: keep survivors (preserving
their health counters and the active round-robin index), append newcomers
with fresh health, drop the rest. Empty snapshots are still ignored so a
transient master outage can't wipe the list.
* wdclient: skip discovery snapshots with no usable addresses
Guard against the defensive case where master returns updates whose
addresses are all empty; reconciling against an empty discovered set
would prune every filer.