Controller's cache isn't thread safe

The Controller's _request_cache attribute isn't used in a thread safe manner. Once upon a time we only added to it so this wasn't an issue, but now that we're doing cache invalidation we need to be more careful. For instance...

# check for cached results
for param in list(lookup_params):
  cache_key = "getconf.%s" % param.lower()

  if cache_key in self._request_cache:
    reply[param] = self._request_cache[cache_key]
    lookup_params.remove(param)

Querying and changing the cache should be under methods that acquire a cache lock.