box: revert the lua space object on the space drop rollback

Before this commit the space rollback had been treated as a new space
creation, so it caused creation of a new space object in the Lua's
box.space namespace. Since the preceding space drop removed the space
object from the namespace, on the space rollback all the Lua users of
the space loosed the track of its changes: the original space object
is never updated anymore.

This is fixed by detecting the space rollback and restoring the old
space object instead of creating a new one.

Closes #9120

NO_DOC=bugfix