Fix uma_zone cache regression

In commit 08835b4e608eddcbd0ce17e3ef7a813d7b239731, we turned off uma
allocation caching when CONF_debug_memory is on because it hid bugs.
Or at least, this is what was *intended*. What the patch actually did
was the opposite - it turned off the cache in the normal compilation!

This caused a deterioration of UDP memcached performance, from 159,000
to 131,000 req/sec. This was the second half of the recent memcached
performance deterioration (see issue #227).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>