EventPublisher was receiving TopicHandlers, which had a couple of
problems:
- ChangeProcessors were being grouped by Topic, but they completely
ignored the topic and were performed on every change
- ChangeProcessors required EventPublisher to be aware of database
changes
By moving ChangeProcesors out of EventPublisher, and having Publish
accept events instead of changes, EventPublisher no longer needs to
be aware of these things.
Handlers is now only SnapshotHandlers, which are still mapped by Topic.
Also allows us to remove the small 'db' package that had only two types.
They can now be unexported types in state.
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.
