Apply hashing to remote storage cache key

Prevents hitting the memcache 250-char key limit when rendering
in cluster setups & using very long metric paths.

This looks like it regressed in f18bb3c7 and since that looks like a squashed merge I can't tell why it was changed during the megacarbon work; presumably it was to update the data used to construct the key. Either way, this change doesn't modify the nature of the key, only ensures it is hashed like all other memcached-using code (eg in {{render/views.py}}).