txn: fix rollback of sub-statements

If space.on_replace callback fails (throws), we must rollback all
statements inserted by the callback before failing and the statement
that triggered the callback while currently we only rollback the last
statement on txn->stmts list. To fix this, let's remember the position
to rollback to in case of failure for each sub statement, similarly to
how we do with savepoints.

Note, there's a comment to txn_rollback_stmt() that says that it doesn't
remove the last statement from the txn->stmts list, just clears it:

        /**
         * Void all effects of the statement, but
         * keep it in the list - to maintain
         * limit on the number of statements in a
         * transaction.
         */
        void
        txn_rollback_stmt()

It isn't going to hold after this patch, because this patch reuses the
savepoint code to rollback statements on failure. Anyway, I haven't
managed to figure out why we would ever need to keep statements on the
list after rollback. The comment is clearly misleading as we don't have
any limits on the number of statements, and even if we had, a statement
counter would suffice. I guess the real reason why we don't delete
statements from the list is that it is simply impossible to do in case
of a singly linked list like stailq, but now it isn't going to be a
problem. So remove the comment and the test case of engine/savepoint
which relies on it.

Needed for #2993
Closes #3020