xrow: add old_tuple, new_tuple to struct request

These fields correspond to the tuple before DML request is executed
(old); and after - result (new). For example let index stores
tuple {1, 1}:
replace{1, 2} -- old == {1, 1}, new == {1, 2}

These fields rather make sense for update operation, which holds
a key and an array of update operations (not the old tuple).

`old_tuple`, `new_tuple` are going to be used as WAL extensions available
in enterprise version. Alongside with it let's reserve 0x2c and 0x2d
Iproto keys for these members.

NO_DOC=<No functional changes>
NO_TEST=<No functional changes>
NO_CHANGELOG=<No functional changes>