temporary_buffer: add make_copy() named constructor
temporary_buffer: add make_copy() named constructor

Add a way to easily copy some memory into a new temporary_buffer.  Useful
where we can't do zero-copy for some reason.

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