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 +234,21 @@ public function collect()
  )
  );
 
+ // With the RequestDataCollector stores the session data and the
+ // session store the collected data in form of stacked data, which
+ // leads to exponential memory usage growth if multiple requests have
+ // their data stacked. Avoid that by temporarily removing the stacked
+ // data from the session.
+ $http = $this->initStackSession();
+ $stack = $http->getSessionValue($this->stackSessionNamespace);
+ $http->deleteSessionValue($this->stackSessionNamespace);
+
  foreach ($this->collectors as $name => $collector) {
  $this->data[$name] = $collector->collect();
  }
 
+ $stack = $http->setSessionValue($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')) {