[GH-196] Make getNamespacedId protected instead of private

This issue is created automatically through a Github pull request on behalf of jnonon:

Url: https://github.com/doctrine/common/pull/196

Message:

We want to extend the cache class provider MemcacheCache.php in order to set a custom namespace. However, because getNamespacedId is a private method, basically a complete rewrite and code duplication of the class is needed.
Is there any reason for this method to be private instead of protected?
Thanks in advace,

--JN