Add getBestBlockId RPC method

[joojis]

Specification: https://github.com/CodeChain-io/codechain/blob/master/spec/JSON-RPC.md#chain_getbestblockid

[joojis]

@kseo Could you review the spec?

[kakao-jun-e]

[URL relocation] Specification: https://github.com/CodeChain-io/codechain/blob/master/spec/JSON-RPC.md#chain_getbestblockid