Enhancement: API request for LIB timestamp

[cc32d9]

This document explains the importance of LIB timestamp for applications:
https://github.com/cc32d9/cc32d9_ideas_for_EOSIO/blob/master/Temporary_history_in_contract_tables.md

Currently LIB timestamp requires two API calls, get_info and get_block, and get_block response is quite bulky sometimes.

The enhancement would bring a cheaper way to retrieve the LIB timestamp: either as a new field in get_info response, or a new API call, like /v1/chain/get_lib which would return the LIB block number, hash and timestamp.