get the default ledger.

GetNextBlockSigner gets the next block signer after block height at timestamp.

GetWinner returns the winner hash and winner type of a block height using sigchain from PoR server.

WinningBlockHash means next Block proposer is signer of historical Block.

The proof of Block proposer validity should exists in previous Block header.

initial version is 0, current version is 1.

Genesis block proposer will propose first 5 blocks.

WinningTxnHash means next Block proposer is a node on signature chain.

ILedgerStore provides func with store package.