This is the interface shared by types that can be used as event data in, for
example, subscriptions via the RPC.
To be compatible with the RPC service, data need to support JSON encoding.
Require this as part of the interface.
Remove the pubsub.Query interface and instead use the concrete query type.
Nothing uses any other implementation but pubsub/query.
* query: remove the error from the Matches method
* Update all usage.
