Improve rpc logic (#6400)

* update rpc imports to be explicit

* avoid exposing HandlerEvent outside RPC

it's unnecessary.

* handle Pongs at RPC handler level

beacon_node/lighthouse_network/src/rpc/methods.rs

@@ -81,7 +81,7 @@ pub struct StatusMessage {
 }
 
 /// The PING request/response message.
-#[derive(Encode, Decode, Clone, Debug, PartialEq)]
+#[derive(Encode, Decode, Copy, Clone, Debug, PartialEq)]
 pub struct Ping {
     /// The metadata sequence number.
     pub data: u64,