gossip: Do not send shutdown message when a node is in unknown status
gossip: Do not send shutdown message when a node is in unknown status

When a replacing node is in early boot up and is not in HIBERNATE sate
yet, if the node is killed by a user, the node will wrongly send a
shutdown message to other nodes. This is because UNKNOWN is not in
SILENT_SHUTDOWN_STATES, so in gossiper::do_stop_gossiping, the node will
send shutdown message. Other nodes in the cluster will call
storage_service::handle_state_normal for this node, since NORMAL and
SHUTDOWN status share the same status handler. As a result, other nodes
will incorrectly think the node is part of the cluster and the replace
operation is finished.

Such problem was seen in replace_node_no_hibernate_state_test dtest:

   n1, n2 are in the cluster
   n2 is dead
   n3 is started to replace n2, but n3 is killed in the middle
   n3 announces SHUTDOWN status wrongly
   n1 runs storage_service::handle_state_normal for n3
   n1 get tokens for n3 which is empty, because n3 hasn't gossip tokens yet
   n1 skips update normal tokens for n3,  but think n3 has replaced n2
   n4 starts to replace n2
   n4 checks the tokens for n2 in storage_service::join_token_ring (Cannot
      replace token {} which does not exist!) or
      storage_service::prepare_replacement_info (Cannot replace_address {}
      because it doesn't exist in gossip)

To fix, we add UNKNOWN into SILENT_SHUTDOWN_STATES and avoid sending
shutdown message.

Tests: replace_address_test.py:TestReplaceAddress.replace_node_no_hibernate_state_test
Fixes: #6436

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla