refactor(pdk) use cjson.new and decode_array_with_array_mt(true)
refactor(pdk) use cjson.new and decode_array_with_array_mt(true)

Instead of switching back and forth with global `cjson`, we instantiate
a new module level `cjson` and set `decode_array_with_array_mt` to true.

Kong/kong,Kong/kong,Mashape/kong,Kong/kong