Implement full Wallet GRPC API

[Rovak]

• [ ] rpc GetAccount (Account) returns (Account)
• [ ] rpc CreateTransaction (TransferContract) returns (Transaction)
• [ ] rpc BroadcastTransaction (Transaction) returns (Return)
• [ ] rpc ListAccounts (EmptyMessage) returns (AccountList)
• [ ] rpc CreateAccount (AccountCreateContract) returns (Transaction)
• [ ] rpc VoteWitnessAccount (VoteWitnessContract) returns (Transaction)
• [ ] rpc CreateAssetIssue (AssetIssueContract) returns (Transaction)
• [ ] rpc ListWitnesses (EmptyMessage) returns (WitnessList)
• [ ] rpc UpdateWitness (WitnessUpdateContract) returns (Transaction)
• [ ] rpc CreateWitness (WitnessCreateContract) returns (Transaction)
• [ ] rpc TransferAsset (TransferAssetContract) returns (Transaction)
• [ ] rpc ParticipateAssetIssue (ParticipateAssetIssueContract) returns (Transaction)
• [x] rpc ListNodes (EmptyMessage) returns (NodeList)
• [ ] rpc GetAssetIssueList (EmptyMessage) returns (AssetIssueList)
• [ ] rpc GetAssetIssueByAccount (Account) returns (AssetIssueList)
• [ ] rpc GetAssetIssueByName (BytesMessage) returns (AssetIssueContract)
• [ ] rpc GetNowBlock (EmptyMessage) returns (Block)
• [ ] rpc GetBlockByNum (NumberMessage) returns (Block)