CACHE_NAME fix

Central server side: make sure caching code is never loaded
Distributed server side: make sure `CACHE_NAME` variable is always defined.

Testing: browse homepage in each of the 4 following conditions:
* `CENTRAL_SERVER=True` and `CACHE_TIME=0`
* `CENTRAL_SERVER=True` and `CACHE_TIME=10000`
* `CENTRAL_SERVER=False` and `CACHE_TIME=0`
* `CENTRAL_SERVER=False` and `CACHE_TIME=10000`

None should give errors :)