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.

node/defaults.go

@@ -45,7 +45,6 @@ var DefaultConfig = Config{
 	HTTPTimeouts:        rpc.DefaultHTTPTimeouts,
 	WSPort:              DefaultWSPort,
 	WSModules:           []string{"net", "web3"},
-	GraphQLPort:         DefaultGraphQLPort,
 	GraphQLVirtualHosts: []string{"localhost"},
 	P2P: p2p.Config{
 		ListenAddr: ":30303",