MM-57152: Get webconn count from the whole cluster (#26813)

We were setting the user status to offline without
checking for connections on other nodes in a cluster.

Now we implement a request-response mechanism for the whole
cluster and we check that before setting a user to offline.

https://mattermost.atlassian.net/browse/MM-57153

```release-note
Fix a bug where the user status would incorrectly be set to offline
without checking for connections in other nodes in an HA cluster.
```

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/testlib/cluster.go

@@ -103,3 +103,7 @@ func (c *FakeClusterInterface) ClearMessages() {
 	defer c.mut.Unlock()
 	c.messages = nil
 }
+
+func (c *FakeClusterInterface) WebConnCountForUser(userID string) (int, *model.AppError) {
+	return 0, nil
+}