Refactor gossip code

[wemeetagain]

Refactors gossip code and adds easier support for supporting future HFs

• A few general fork helpers, the IForkInfo interface, config.getForkInfoRecord(), and computeForkNameFromForkDigest
• Remove outer Gossip wrapper, all logic is on Eth2Gossipsub, a class that extends Gossipsub.
• Rework gossip topic typing, add type GossipTopic that supports referencing a topic in a friendly way
• Generally, validation functions and gossip event handlers are added on initialization, removed on destruction. (rather than added/removed during subscription/unsubscription)
• Subscription/unsubscription is toggled by starting/stopping the sync gossip handler, attestation collector, and interopSubnetTask
• Generally, ForkDigest event handlers can/should be replaced with IForkName event handlers.
  • in many cases, the way to handle events is known ahead of time in a per-fork manner
  • fork digest is not immediately useful, rather, its derivative of the fork version/name
  • since genesisValidatorsRoot is a constant, we can code in terms of IForkName and derive a ForkDigest if/when/where its needed
• Adds a few metrics:
  

Fixes #2083

[codeclimate]

Code Climate has analyzed commit dcd7d4a and detected 11 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	5
Duplication	6

View more on Code Climate.