gossiper: remove uses of get_local_gossiper for its rpc server
gossiper: remove uses of get_local_gossiper for its rpc server

Initialization happens in the gossiper itself, so we can capture
'this'. If we need to move to shard 0, use sharded::invoke_on() to
get the local instance.

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