Add EligibleDelegateStore to DPoS module

[shuse2]

Description

• Add EligibleDelegateStore as described in LIP-0057
• Add getKey(address: Buffer, newWeight: bigint) function to the store
• Add getTop(count: number) => { weight: bigint, address: Buffer }[] function to the store.
  • it should be iterated in reverse order
• Add getAll() { weight: bigint, address: Buffer }[] function to the store

Acceptance Criteria

• All functions are unit tested with in memory store