Compatibility with Doctrine's MemcachedCache

[karlbuckland]

It looks like the keys generated by FindObjectsQueryFactory in the compute() method (for example) contain spaces in the filterString and therefore the cache $key, which Doctrine's MemcachedCache class does not allow:

https://github.com/doctrine/cache/blob/master/lib/Doctrine/Common/Cache/MemcachedCache.php

See the validateCacheId() method.

Sidenote: I am triggering this problem by performing a $this->find() method call within a Dao object, such as:
return $this->find("user_agent IN (:user_agent)", [ 'user_agent' => implode(',', $user_agent) ]);

In this example, this part of the query ends up in the filterString ("user_agent IN (:user_agent)") and that is where the spaces are.

When tested with ArrayCache or ApcuCache it works fine.