Use a separate lock for subscriptions.ByToken to allow it to happen synchronously
in the commit flow.
This removes the need to create a new txn for the goroutine, and removes
the need for EventPublisher to contain a reference to DB.
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