Before this change, if the user changed their password or public-key
and the auth proxy script returned updated config parameters for the
backend (eg a rotated api_key) rclone would continue to re-use the old
backend with the old config parameters out of the fscache.
This was because both the VFS cache and the fs/cache key were derived
from the user name only, so a change in the user's password or
public-key did not invalidate the cached backend.
Fix this by deriving the cache key from the user plus a hash of the
password/public-key, so a credential change forces a fresh backend.
The hash uses a per-process random HMAC key so the fragment that
appears in logs cannot be brute-forced offline.
df9935d71e