Define storage for saving info about legacy chain brackets

[ishantiw]

Description

• Create a new DB_KEY_LEGACY_BRACKETS for storing legacy chain brackets
• Each bracket is identified by Buffer.concat([DB_KEY_LEGACY_BRACKETS, bracket.snapshotBlockID]) with value as

interface LegacyChainBracketInfo { 
  startHeight: number;
  snapshotBlockHeight: number;
  lastBlockHeight: number;
}

• In LegacyChainHandler.init, if there is no key exists for a snapshotBlockID we initialize as below,

key:

config.bracket[index].snapshotBlockID

value:

{
  startHeight: config.bracket[index].startHeight,
  snapshotBlockHeight: config.bracket[index].snapshotBlockHeight,
  lastBlockHeight:  config.bracket[index].snapshotBlockHeight
}

• Implement getLegacyChainBracketInfo(snapshotBlockID): Promise<LegacyChainBracket>
• Implement setLegacyChainBracketInfo(snapshotBlockID: Buffer, bracketInfo: LegacyChainBracket): Promise<void>

Acceptance Criteria

• Should have unit test corresponding to it