This makes several updates to support the blockupdates plugin.
I had to update several hooks that were using the wrong types, and
provide a way to get event.Feed objects into plugins without importing
event.Feed (which I did by having the plugin loader make them
available).
This does some dark magic with reflect to enable plugins to offer
subscriptions without requiring them to use the rpc.Subscriptions
import.
Basically, plugin services can have a channel as a return value,
and the shim will pull items off of that chanel and send them to
the notifier. It makes sure that context.Done() will fire when
the user disconnects, and will shut everything down if the channel
closes.
