SWIM: once a node is dead, set a TTL for the tombstone and remove it later

[ktoso]

~We must schedule a periodic cleanup of the dead members otherwise the list would grow indefinitely~

Dead's we remove today, it's just unreachables that stay around.

We may want to allow "keep dead nodes as tombstones".

It depends on the runtime if it needs this feature or not, SWIMNIO would need it for example to be a serious impl.