chunked_fifo: implement back()
chunked_fifo: implement back()

Useful for modifying an element immediately after push_back().

cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,dreamsxin/seastar,kjniemi/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,kjniemi/seastar,kjniemi/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar