steam-api: do not directly access memory within json_values

Currently, the functions: steam_api_friends(), steam_api_poll(), and
steam_api_summaries() return GSLists that point directly to memory
allocated within json_values. While this is more efficient, it is rather
ugly. Moreover, with certain versions/architectures of GCC/glibc, issues
have come up during run-time.
steam-api: retrieve and process message timestamps