When a node is removed from the _live_endpoints list directly, e.g., a
node being decommissioned, it is possible the node might not be marked
as down in gossiper::failure_detector_loop_for_node loop before the loop
exits. When the gossiper::failure_detector_loop loop starts again, the
node will not be considered because it is not present in _live_endpoints
list any more. As a result, the node will not be marked as down though
gossiper::failure_detector_loop_for_node loop.
To fix, we mark the nodes that are removed from _live_endpoints
lists as down in the gossiper::failure_detector_loop loop.
Fixes#8712Closes#8770
0665d9c346