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. 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.

The same RPC endpoint is also about to be merged for ETC's
go-ethereum: ethereumproject/go-ethereum#336

…c/eth-chainId

u64 returns decimal integer, and there seems to be no type called
U64. So here I use U256 to return the hex integer.

Before EIP155 fork number, chainID should be null.

This makes it consistent that all chain ids returned are hex string.