Also store the index in Changes instead of the Txn.
This change is in preparation for movinng EventPublisher to the stream package, and
making handleACLUpdates async once again.
Also remove secretHash, which was used to hash tokens. We don't expose
these tokens anywhere, so we can use the string itself instead of a
Hash.
Fix acl_events_test.go for storing a structs type.
Daniel Nephin