When ForwardingCacheBackend was first created, it had to work around

some assumptions in Django where a cache backend could be created but

then never replaced.
Modern Django manages cache backends through a mutable registry,

removing much of the need for ForwardingCacheBackend. However, it's

still a useful API.
In an effort to keep the logic simple, and to remove any

threading-related requirements in the implementation, this change

updates the backend to just look up the named backend on demand, rather

than storing an instance of it. This should avoid any issues where a

stale backend could ever be in use, and allows Django's connection

management to handle any threading responsibility.
There's also a fix in the code for applying siteconfig settings. The old

code assumed that django.core.cache.cache was an instance of the

backend, but it's not anymore. So instead, we now optimistically call

the function we need on it, ignoring any attribute errors if not found.

All Djblets and Review Board unit tests pass.
Tested this with a local server. Verified that when I changed settings,

the cache updated.