This follows the same model as we did in the p2p package.
Rework the indexer service constructor to take a struct of arguments,
that makes it easier to construct the optional settings.
Deprecate but do not remove the existing constructor.
Clean up node initialization a little bit.
This is part of the work described by #7156.
Remove "unbuffered subscriptions" from the pubsub service.
Replace them with a dedicated blocking "observer" mechanism.
Use the observer mechanism for indexing.
Add a SubscribeWithArgs method and deprecate the old Subscribe
method. Remove SubscribeUnbuffered entirely (breaking).
Rework the Subscription interface to eliminate exposed channels.
Subscriptions now use a context to manage lifecycle notifications.
Internalize the eventbus package.
