box: replace port_tuple with port_c everywhere

Port_tuple is exactly the same as port_c, but is not able to store
raw MessagePack. In theory it sounds like port_tuple should be a
bit simpler and therefore faster, but in fact it is not.
Microbenchmarks didn't reveal any difference. So port_tuple is no
longer needed, all its functionality is covered by port_c.

Follow up #4641