NOJIRA-1070 Rewrite ExternalCache

Turns out that one-cache-object per namespace wasn’t such  a good idea
after all because memcached and apc don’t have inherent support for
namespaces. So we have to do the namespacing by prefixing the keys.
That’s not a problem except we now can’t wipe/flush specific namespaces
anymore as these adapters don’t have a way to iterate over all keys.
They really kept it as simple as possible.

The tests now pass with all backends. The new file backend has the
typical file permission problems, i.e. when the cache is created by the
web server, you can’t access/write it as normal user. But I guess that
problem existed with Zend as well.