The EventPublisher is the central hub of the PubSub system. It is toughly coupled with much of
stream. Some stream internals were exported exclusively for EventPublisher.
The two Subscribe cases (with or without index) were also awkwardly split between two packages. By
moving EventPublisher into stream they are now both in the same package (although still in different files).
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.
Make topicRegistry use functions instead of unbound methods
Use a regular memDB in EventPublisher to remove a reference cycle
Removes the need for EventPublisher to use a store
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