Use Memcache and clean up caching setup.

- remove django-cache-machine

- removed secondary cache alias again

- use Django's cached_property decorator instead of own

- updated constance's cache setting to last format

- django-memcached-hashring for consistent hashing (based on python-memcached)

Fixes #1780.
Merge pull request #1853 from jezdez/cache-cleanup

[DO NOT MERGE] Use Memcache and clean up caching setup.