[TECHDEBT] Consolidate & encapsulate `ValidatorMap` logic

[Olshansk]

Objective

Encapsulate the concept of a ValidatorMap only to the consensus module

Origin Document

This discussion in #178:

Goals

• Remove all occurrences/references to the ValidatorMap throughout the codebase
• Only maintain knowledge/understanding o the ValidatorMap in the consensus module

Deliverable

• Do a grep for ValidatorMap throughout the whole codebase and it should only show up in the consensus module
• A single PR refactoring / cleaning up this goal

Non-goals / Non-deliverables

• Adding new functionality

General issue deliverables

• Update the appropriate CHANGELOG
• Update any relevant READMEs (local and/or global)
• Update any relevant global documentation & references

Testing Methodology

• All tests: make test_all
• LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md

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Creator: @Olshansk

[Olshansk]

This is related to #271 in the greater scope/vision of having dynamic validator sets as they change with each height.

#271 is scoped to only P2P and doing structured gossip amongst the validator nodes (for now). In general, P2P does care who the nodes are but simply gossips amongst the actors it is given.

This ticket is scoped to consensus (techdebt cleanup) to make sure the concept of a "validator set" only exists in consensus.

To keep in mind

• As this is done (in isolation), the goal is to think how to implement dynamic additions of validators & P2P actors.
• Further, thinking through how LocalNet can be designed/updated to dynamically add new validators is a non-trivial amount of work that's of high importance.

[deblasis]

@jessicadaugherty heads up I am a bit behind schedule with #331 and #203 (ValidatorMap related work). I was supposed to be done yesterday but I think I'll need another day or two to have them both under review.