memtx: handle all statements related to the space on DDL

When DDL happens, we remove statements of concurrent transactions
from MVCC. When removing statements, we set their `engine_savepoint` to
`NULL` so that they won't be rolled back because we've already handled
them.

However, we remove statements only from stories, and not all statements
can be accessed in this way. For example, when we have several delete
statements of one story, and one of them gets prepared, others are
unlinked. It leads to use-after-free, but it's read-only and doesn't
affect anything, so only ASAN can catch it. It happens when the
statement is being rolled back in `memtx_engine_rollback_statement`:
we check if `space->upgrade` is not `NULL` (space can be already deleted)
but this check affects instruction flow only if `stmt->new_tuple != NULL`
and in our case that's not so. Anyway, let's iterate over all statements
of all transactions and remove savepoints for ones related to the space
that is being invalidated. It takes more time, but anyway, we are doing
DDL that is heavy, so it doesn't really matter.

Along the way, the commit removes helper `memtx_tx_history_remove_stmt`
and all its helpers because they are not needed anymore. This helper
unlinks added story from history chain, links all its delete statements
to the previous story, if any, unlinks the statement from related stories
and sets `engine_savepoint` to `NULL`. Since we already do all of this
things except for unlinking statements from stories, let's simply call
`memtx_tx_story_unlink_added[deleted]_by` instead. This change makes the
code much more straightforward.

Closes #10146

NO_DOC=bugfix

(cherry picked from commit ac112b73192ad96271a02ee85dba3e9737fdaa9d)