node: refactor package node (#21105)

This PR significantly changes the APIs for instantiating Ethereum nodes in
a Go program. The new APIs are not backwards-compatible, but we feel that
this is made up for by the much simpler way of registering services on
node.Node. You can find more information and rationale in the design
document: https://gist.github.com/renaynay/5bec2de19fde66f4d04c535fd24f0775.

There is also a new feature in Node's Go API: it is now possible to
register arbitrary handlers on the user-facing HTTP server. In geth, this
facility is used to enable GraphQL.

There is a single minor change relevant for geth users in this PR: The
GraphQL API is no longer available separately from the JSON-RPC HTTP
server. If you want GraphQL, you need to enable it using the
./geth --http --graphql flag combination.

The --graphql.port and --graphql.addr flags are no longer available.

cmd/wnode/main.go

@@ -221,8 +221,7 @@ func initialize() {
 		MaxMessageSize:     uint32(*argMaxSize),
 		MinimumAcceptedPOW: *argPoW,
 	}
-
-	shh = whisper.New(cfg)
+	shh = whisper.StandaloneWhisperService(cfg)
 
 	if *argPoW != whisper.DefaultMinimumPoW {
 		err := shh.SetMinimumPoW(*argPoW)
@@ -433,7 +432,7 @@ func run() {
 		return
 	}
 	defer server.Stop()
-	shh.Start(nil)
+	shh.Start()
 	defer shh.Stop()
 
 	if !*forwarderMode {