box: fix space:bsize() handling on space alter

During building an index in background, some transaction can perform a dml
request that affects space size (e.g. a replace), but the size will remain
the same, because bsize is moved from the old space to the new space in
memtx_space_prepare_alter() prior to space_execute_dml(). Fix this issue by
calling space_finish_alter() in alter_space_do().
In fact, this patch partially reverts commit 9ec3b1a4 ("alter: zap
space_vtab::commit_alter").

NO_DOC=bugfix

Closes #9247