Provide array cache implementation

doctrine/cache 2.0 no longer provides implementations of caches. It provides only interfaces.
We need only PHP array cache so I chose to implement this simple cache instead of introducing
new cache library.
[MAILPOET-3892]

lib/Doctrine/ConfigurationFactory.php

@@ -3,7 +3,6 @@
 namespace MailPoet\Doctrine;
 
 use MailPoet\Doctrine\Annotations\AnnotationReaderProvider;
-use MailPoetVendor\Doctrine\Common\Cache\ArrayCache;
 use MailPoetVendor\Doctrine\Common\Proxy\AbstractProxyFactory;
 use MailPoetVendor\Doctrine\ORM\Configuration;
 use MailPoetVendor\Doctrine\ORM\Mapping\Driver\AnnotationDriver;