Refactor GossipRouter: extract GossipRouterBuilder

[Nashatyrev]

This is follow up to #247

In the very initial version I've made an attempt to invent a new Kotlin specific pattern which combines both builder and its resulting class entities. It should in theory make the code smaller and supporting efforts cheaper.

But it finally failed to be both a builder pattern and any kind of sustainable code. With GossipRouter becoming more complex class that approach is turning into error-prone spaghetti code.

This PR extracts GossipRouterBuilder with making GossipRouter properties final (where appropriate), which looks more like a traditional pattern for Teku code and should be less error-prone and more sustainable

Also:

• Move property declarations right after primary constructor (which may also declare properties) in changed classes
• Refactor AbstractRouter.addPeerWithDebugHandler() workaround
• Remove pubsub interface var properties
• Adjust tests

[tbenr]

LGTM.