Implement blueprint delete-box-return-tuple

In DELETE, support flag BOX_RETURN_TUPLE.

A community request from Alexandre Kalendarev.

In the new DELETE (21) command, one can specify
delete flags. If BOX_RETURN_TUPLE is set, return
the old (deleted) tuple to the client.

Update doc/box-protocol.txt to reflect the new
(21) DELETE and its extended response format.