[RFC-201/205]: EVM-580-Update validator set in stake manager

[goran-ethernal]

Description

This PR introduces a new concept: stakeManager.

stakeManager does two things:

• On each block insert (either from syncer or consensus) reads all Transfer events from the ValidatorSet contract on child chain and saves them to the stakeChange bucket in boltDB for the epoch in which occurred (Transfer events carry information about stake change of validators in given epoch).
• At the end of epoch, it will calculate and provide an updated validator set based on all Transfer events that happened in the previous epoch (epoch that is currently ending).

PR also added UnitTests for stakeManager and its store that saves the Transfer events to boltdb.

IMPORTANT NOTE: e2e tests will fail until all client logic is implemented. Fixing of e2e tests will be the last PR for RFC-201/205 saga.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually