Rethink NettyServer.channelsStream implementation

[ericsson49]

NettyServer uses UnicastProcessor as a publisher of incoming connections. Thus it supports one subscriber only. This is a problem, if one wants to implement beaconchain_peers metric, for example. As a part of metrics implementation (#178), I temporarily attached ReplayProcessor to UnicastProcessor, and used former as channels stream (by analogy to ConnectionManager client connections stream).
While it's working, the implementation should be rewised.