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.

I had to do this, since ```IndexRegistry has type parameter, and this change introduces dependency form IndexRegistry to EurekaRegistry. You can see that ```LeasedInstanceRegistry``` is not generic.
With ```IndexProvider``` interface I have proposed above, we could revert this change.