gossip: Speed up wait for gossip settle
gossip: Speed up wait for gossip settle

In a large cluster, a node would receive frequent and periodic gossip
application state updates like CACHE_HITRATES or VIEW_BACKLOG from peer
nodes. Those states are not critical. They should not be counted for the
_msg_processing counter which is used to decide if gossip is settled.

This patch fixes the long settle on every restart issue reported by
users.

Refs #10337

Closes #10892

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