qsync: handle async txns right during CONFIRM

It is possible that a new async transaction is added to the limbo
when there is an in-progress CONFIRM WAL write for all the pending
sync transactions.

Then when CONFIRM WAL write is done, it might see that the limbo
now in the first place contains an async transaction not yet
written to WAL. A suspicious situation - on one hand the async
transaction does not have any blocking sync txns before it and
can be considered complete, on the other hand its WAL write is not
done and it is not complete.

Before this patch it resulted into a crash - limbo didn't consider
the situation possible at all.

Now when CONFIRM covers a not yet written async transactions, they
are removed from the limbo and are turned to plain transactions.

When their WAL write is done, they see they no more have
TXN_WAIT_SYNC flag and don't even need to interact with the limbo.

It is important to remove them from the limbo right when the
CONFIRM is done. Because otherwise their limbo entry may be not
removed at all when it is done on a replica. On a replica the
limbo entries are removed only by CONFIRM/ROLLBACK/PROMOTE. If
there would be an async transaction in the first position in the
limbo queue, it wouldn't be deleted until next sync transaction
appears.

This replica case is not possible now though. Because all synchro
entries on the applier are written in a blocking way. Nonetheless
if it ever becomes non-blocking, the code should handle it ok.

Closes #6057