Add RPC eth_chainId for querying the current blockchain chain ID

[tcz001]

Preamble

EIP: EIP694
Title: Add RPC eth_chainId for querying the current blockchain chain ID
Type: Standard Track
Category: Interface
Status: Draft
Created: 2017-08-21

Simple Summary

Add RPC eth_chainId for querying the current blockchain chain ID

Abstract

Currently although we can use net_version RPC call to get the current network ID, there's no RPC for querying the chain ID. This makes it impossible to determine the current actual blockchain using the RPC.

Motivation

An ETH/ETC client can accidentally connect to an ETC/ETH RPC endpoint without knowing it unless it tries to sign a transaction or it fetch a transaction that is known to have signed with a chain ID. This has since caused trouble for application developers, such as MetaMask, to add multi-chain support.

Specification

As Parity PR and ETC go-ethereum PR, we can achieve cross-client compatible RPC API by using the same approach.
As described in ethereumproject/go-ethereum PR:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x3d"
}

[whilei]

I have submitted this a PR per the README instructions - #695.