rpc: make server statistics per server, not per connection
rpc: make server statistics per server, not per connection

Connections are anonymous so their statistics are not interesting; also
they disappear once the connection is broken.

Fix by making the statistics per server.

Note this changes the statistics API.

syuu1228/seastar,cloudius-systems/seastar,sjperkins/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,kjniemi/seastar,kjniemi/seastar,scylladb/seastar,avikivity/seastar,kjniemi/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,sjperkins/seastar,sjperkins/seastar,dreamsxin/seastar