Simplify Session's RWMutex

The Session struct owns its lock (an RWMutex) and does not share it
with any other code. It can therefore be stored in Session by value.

This eliminates a layer of pointer indirection, and simplifies the code
slightly.

TEST=$ go test -race ./...

Author: TheSven73

melody.go

@@ -2,7 +2,6 @@ package melody
 
 import (
 	"net/http"
-	"sync"
 
 	"github.com/gorilla/websocket"
 )
@@ -161,7 +160,6 @@ func (m *Melody) HandleRequestWithKeys(w http.ResponseWriter, r *http.Request, k
 		outputDone: make(chan struct{}),
 		melody:     m,
 		open:       true,
-		rwmutex:    &sync.RWMutex{},
 	}
 
 	m.hub.register(session)

session.go

@@ -18,7 +18,7 @@ type Session struct {
 	outputDone chan struct{}
 	melody     *Melody
 	open       bool
-	rwmutex    *sync.RWMutex
+	rwmutex    sync.RWMutex
 }
 
 func (s *Session) writeMessage(message envelope) {