Provide PSR array cache and switch annotation reader to it

The annotation reader was using CachedReader which has been deprecated so we need to switch to PsrCachedReader
Since 2.0 doctrine/cache no longer provides implementations of caches so I also needed to implement a simple PSR array cache.
[MAILPOET-3892]

lib/Doctrine/PSRArrayCache.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace MailPoet\Doctrine;
+
+use MailPoetVendor\Psr\Cache\CacheItemInterface;
+use MailPoetVendor\Psr\Cache\CacheItemPoolInterface;
+
+class PSRArrayCache implements CacheItemPoolInterface {
+  /** @var mixed[] */
+  private $cache = [];
+
+  /**
+   * @inheritDoc
+   */
+  public function getItem($key) {
+    if (!is_string($key)) {
+      throw new PSRCacheInvalidArgumentException('Invalid key');
+    }
+    if (!$this->hasItem($key)) {
+      return new PSRCacheItem($key, false);
+    }
+    return new PSRCacheItem($key, $this->cache[$key]);
+  }
+
+  public function getItems(array $keys = []) {
+    return array_map([$this, 'getItem'], $keys);
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function hasItem($key) {
+    return array_key_exists($key, $this->cache);
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function clear() {
+    $this->cache = [];
+    return true;
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function deleteItem($key) {
+    if (!is_string($key)) {
+      throw new PSRCacheInvalidArgumentException('Invalid key');
+    }
+    unset($this->cache[$key]);
+    return true;
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function deleteItems(array $keys) {
+    try {
+      array_map([$this, 'deleteItem'], $keys);
+    } catch (PSRCacheInvalidArgumentException $e) {
+      return false;
+    }
+    return true;
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function save(CacheItemInterface $item) {
+    $this->cache[$item->getKey()] = $item->get();
+    return true;
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function saveDeferred(CacheItemInterface $item) {
+    return $this->save($item);
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public function commit() {
+    return true;
+  }
+}