Reduce memory usage for consecutive requests with stacked data

Currently, the session stores stacked data, which in turn might contain
the session data if a RequestCollector is active. This leads to
exponential growth in memory usage and rapid performance degradation if
multiple requests add stacked data before the next time the debug bar is
rendered.

We can avoid this be temporarily removing the stacked data from the
session while collection data.

src/DebugBar/DebugBar.php

@@ -234,10 +234,25 @@ public function collect()
             )
         );
 
+        // The RequestDataCollector stores a copy of the session data and the
+        // session stores the collected data when it gets stacked. This leads
+        // to exponential memory usage growth if multiple requests have
+        // their data stacked before the stacked data is rendered. We can avoid
+        // that by temporarily removing the stacked data from the session.
+        $http = $this->getHttpDriver();
+        if ($http->isSessionStarted()) {
+            $stack = $http->getSessionValue($this->stackSessionNamespace);
+            $http->deleteSessionValue($this->stackSessionNamespace);
+        }
+
         foreach ($this->collectors as $name => $collector) {
             $this->data[$name] = $collector->collect();
         }
 
+        if ($http->isSessionStarted()) {
+            $http->setSessionValue($this->stackSessionNamespace, $stack);
+        }
+
         // Remove all invalid (non UTF-8) characters
         array_walk_recursive($this->data, function (&$item) {
                 if (is_string($item) && !mb_check_encoding($item, 'UTF-8')) {