hint_sender decides whether to send a hint directly to its destination
or to re-mutate from scratch based on token_metadata::is_leaving(),
which only checks whether the *host* is leaving the cluster. When a
tablet is dropped from a host due to RF reduction (RF--), the host
is still alive and is_leaving() returns false, so hint_sender sends
directly to a replica that will no longer own the data -- effectively
losing the hint.
Switch to the new ermp->is_leaving(host, token) which is tablet-aware.
When the destination's tablet is being migrated away *and* there are
pending endpoints, send directly (the pending endpoints will receive
the data via streaming); otherwise fall through to the re-mutate path
so all current replicas receive the mutation.
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