During union of two migration groups
(as a result of changing
write_requested() is called
only on one
object from the first group.
Because of archivation of the
write_requested() must be also called
any object from the second group.
It is not necessary
to call write_requested(),
but it is sufficient to call only some
from object manager, which gets pointer to
object from the second group (so the object
can enumerate the state of the second group).