use a separate (memory-based) cache for sessions

This addresses issue #265.

Our sessions cache is using the same file-based cache that we use for page caching.  This is "bad": file cache is slow, and if CACHE_TIME is set to zero, we will get different behavior for the session cache.

The change:
* Define a separate memory-based cache.

Testing (I've done all these, and more):
* Pull, log in as a student, close browser, re-open.  Try with `CACHE_TIME=0` in local_settings.py