cache: Remove KEY_PREFIX from keys returned by cache_get_many.

Otherwise code paths that use those keys, like get_old_messages, will
incorrectly use the prefix-included keys.

This bug in our KEY_PREFIX system results in our memcached caching for
get_old_messages always missing.

(imported from commit 506c13e06d6f266596ead0b381c324c256e576c3)

zephyr/lib/cache.py

@@ -133,7 +133,7 @@ def cache_get_many(keys, cache_name=None):
         cache_backend = get_cache(cache_name)
     ret = cache_backend.get_many(keys)
     memcached_stats_finish()
-    return ret
+    return dict([(key[len(KEY_PREFIX):], value) for key, value in ret.items()])
 
 def cache_set_many(items, cache_name=None, timeout=None):
     new_items = {}