Add a batch command allowing to invalidate a whole namespace

When a cache system (say Memcached farm) is used to store multiple namespaces, rather than flushing the whole cache, it can be relevant to invalidate only one namespace.

This is the purpose of the deleteAll method implemented by all cache providers in Doctrine Cache (see Doctrine\Common\Cache\CacheProvider abstract class).

Of course, this is only useful when the cache storage is reachable by both a CLI command and the PHP SAPI used to treat pages/webservices. This excludes APC for instance which stores its data in a shared memory available only in the same context.
But this can be really interesting for shared cache storage like memcache or Couchbase.