#462 made StreamProducer/StreamConsumer not generic on the Store or pub/sub type.

Right now the interface assumes the stream is heterogenous so the type yielded by a consumer is Proxy[Any]. It may make more sense to assume that StreamConsumer is bound to a type T such that Proxy[T] is yielded. In other words, assume that streams are homogenous by default and allow users to type them as StreamConsumer[Any] if it's a heterogenous stream.