Support eth_chainId

[axic]

See ethereum/EIPs#695 and openethereum/parity-ethereum#6329 (merged into parity).

This would be useful for the upcoming ewasm testnet & tooling around it.

[halfalicious]

I'd like to take this one 😀 A couple of quick questions:

• Is there any documentation for regenerating libweb3jsonrpc\Ethface.h after I've added the new chainId function to eth.json? I've tried running jsonrpcstub eth.json --cpp-server=dev::rpc::EthFace but there are some differences between my generated file and the existing one (e.g. my EthFace class extends jsonrpc::AbstractServer<EthFace> rather than ServerInterface<EthFace>). Apologies if this is a naive question, I haven't used jsonrpcstub before.
• Same goes for tests\unittests\libweb3jsonrpc\WebThreeStubClient.h - is there any documentation for how to properly regenerate this? Additionally, which json file do I need to update? I don't see one in tests\unittests\libweb3jsonrpc

Thank you!