rpc: implement websockets with github.com/gorilla/websocket (#19866)

* rpc: implement websockets with github.com/gorilla/websocket

This change makes package rpc use the github.com/gorilla/websocket
package for WebSockets instead of golang.org/x/net/websocket. The new
library is more robust and supports all WebSocket features including
continuation frames.

There are new tests for two issues with the previously-used library:

  - TestWebsocketClientPing checks handling of Ping frames.
  - TestWebsocketLargeCall checks whether the request size limit is
    applied correctly.

* rpc: raise HTTP/WebSocket request size limit to 5MB

* rpc: remove default origin for client connections

The client used to put the local hostname into the Origin header because
the server wanted an origin to accept the connection, but that's silly:
Origin is for browsers/websites. The nobody would whitelist a particular
hostname.

Now that the server doesn't need Origin anymore, don't bother setting
one for clients. Users who need an origin can use DialWebsocket to
create a client with arbitrary origin if needed.

* vendor: put golang.org/x/net/websocket back

* rpc: don't set Origin header for empty (default) origin

* rpc: add HTTP status code to handshake error

This makes it easier to debug failing connections.

* ethstats: use github.com/gorilla/websocket

* rpc: fix lint

rpc/json.go

@@ -141,6 +141,11 @@ type Conn interface {
 	SetWriteDeadline(time.Time) error
 }
 
+type deadlineCloser interface {
+	io.Closer
+	SetWriteDeadline(time.Time) error
+}
+
 // ConnRemoteAddr wraps the RemoteAddr operation, which returns a description
 // of the peer address of a connection. If a Conn also implements ConnRemoteAddr, this
 // description is used in log messages.
@@ -165,12 +170,10 @@ type jsonCodec struct {
 	decode     func(v interface{}) error // decoder to allow multiple transports
 	encMu      sync.Mutex                // guards the encoder
 	encode     func(v interface{}) error // encoder to allow multiple transports
-	conn       Conn
+	conn       deadlineCloser
 }
 
-// NewCodec creates a new RPC server codec with support for JSON-RPC 2.0 based
-// on explicitly given encoding and decoding methods.
-func NewCodec(conn Conn, encode, decode func(v interface{}) error) ServerCodec {
+func newCodec(conn deadlineCloser, encode, decode func(v interface{}) error) ServerCodec {
 	codec := &jsonCodec{
 		closed: make(chan interface{}),
 		encode: encode,
@@ -183,12 +186,14 @@ func NewCodec(conn Conn, encode, decode func(v interface{}) error) ServerCodec {
 	return codec
 }
 
-// NewJSONCodec creates a new RPC server codec with support for JSON-RPC 2.0.
+// NewJSONCodec creates a codec that reads from the given connection. If conn implements
+// ConnRemoteAddr, log messages will use it to include the remote address of the
+// connection.
 func NewJSONCodec(conn Conn) ServerCodec {
 	enc := json.NewEncoder(conn)
 	dec := json.NewDecoder(conn)
 	dec.UseNumber()
-	return NewCodec(conn, enc.Encode, dec.Decode)
+	return newCodec(conn, enc.Encode, dec.Decode)
 }
 
 func (c *jsonCodec) RemoteAddr() string {