A core's pubsub registry works through reloads.

Adds a protocol and records for subscriptions and their registries.
Pubsub registries are a Service. Subscriptions come in two flavors:
persistent and transient. Transient subscriptions, like those used in
streams, are wiped when the config is reloaded. Persistent
subscriptions, like the ones used by servers, survive the sweeping
process. Since the core now considers pubsub registries equivalent
during transition!, the same registry will be used across reloads.

This means the dash can continue to receive updates for queries against
an index, even through reloads. Closes #88.