MM-61225: Revert session pooling (#28901)

This originated from https://github.com/mattermost/mattermost/issues/15249.

However, the original idea was discarded https://github.com/mattermost/mattermost/issues/15249#issuecomment-709713065
as being too complicated to implement. Then I had another
idea to implement it just for session objects.

My thinking was that since every single request allocates a new
session struct, it would be good to use a sync.Pool for that.

However, 4 years later, now we know that the primary bottleneck
in app performance comes from websocket event marshalling.
Therefore, while it would be good to do this, it is difficult
to do it correctly (as shown by the numerous racy tests).

Hence, reverting this.

```release-note
NONE
```

server/channels/app/platform/web_conn.go

@@ -194,6 +194,15 @@ func (ps *PlatformService) PopulateWebConnConfig(s *model.Session, cfg *WebConnC
 
 // NewWebConn returns a new WebConn instance.
 func (ps *PlatformService) NewWebConn(cfg *WebConnConfig, suite SuiteIFace, runner HookRunner) *WebConn {
+	userID := cfg.Session.UserId
+	session := cfg.Session
+	if cfg.Session.UserId != "" {
+		ps.Go(func() {
+			ps.SetStatusOnline(userID, false)
+			ps.UpdateLastActivityAtIfNeeded(session)
+		})
+	}
+
 	// Disable TCP_NO_DELAY for higher throughput
 	var tcpConn *net.TCPConn
 	switch conn := cfg.WebSocket.UnderlyingConn().(type) {
@@ -245,23 +254,9 @@ func (ps *PlatformService) NewWebConn(cfg *WebConnConfig, suite SuiteIFace, runn
 		remoteAddress:      cfg.RemoteAddress,
 		xForwardedFor:      cfg.XForwardedFor,
 	}
-
-	wc.SetSession(&cfg.Session)
-	userID := cfg.Session.UserId
-	if userID != "" {
-		// UpdateLastActivityAtIfNeeded might block if the Hub is busy.
-		// Create a goroutine to avoid blocking the creation of the websocket connection.
-		ps.Go(func() {
-			ps.SetStatusOnline(userID, false)
-			session := wc.GetSession()
-			if session != nil {
-				ps.UpdateLastActivityAtIfNeeded(*session)
-			}
-		})
-	}
-
 	wc.Active.Store(cfg.Active)
 
+	wc.SetSession(&cfg.Session)
 	wc.SetSessionToken(cfg.Session.Token)
 	wc.SetSessionExpiresAt(cfg.Session.ExpiresAt)
 	wc.SetConnectionID(cfg.ConnectionID)
@@ -393,8 +388,6 @@ func (wc *WebConn) GetSession() *model.Session {
 
 // SetSession sets the session of the connection.
 func (wc *WebConn) SetSession(v *model.Session) {
-	// Clone the session first as WebConn takes ownership of the object
-	// and the web.Hub will return it to the [sync.Pool] once the WebConn gets removed.
 	if v != nil {
 		v = v.DeepCopy()
 	}