memtx: fix use-after-free in mvcc on ddl

When space is being altered, `memtx_tx_space_on_delete` is called - it
deletes all the stories associated with the old schema. However, before
deleting a story, its `reader_list` member is not unlinked from the list
so other nodes can still access this memory. The commit fixes this
problem and adds an assertion that checks if story is always unlinked
from reader list when is being deleted.

Part of #10146

NO_CHANGELOG=later
NO_DOC=bugfix

(cherry picked from commit a32f56dfbb4b56b410ac376fce079613cac0ccb6)