2.x Implement composite interest index as an aggregate of atomic indexes.

I still keep InterestNotificationMultiplexer, as it allows us to dynamically adjust notification stream. During interest upgrade, we do not want to resend data of atomic interests that are still part of the upgraded interest set. Swapping composite interest subscriptions would sent everything again.

Index extends rx.Subject which does not make sense for the composite interest. What index really is, it is an observable of events to which we can subscribe. hence the change. Index class is an internal implementation detail.