wal: correctly assign entry->res to the signature of the last row of the transaction

vinyl expects that entry->res contains a correct signature of the
transaction. We used to sloppily assign it to some non-zero signature
of the last committed transaction in the batch in case of success.
Vinyl expects an exact value, growing monotonically for each next
transaction.

Optimistically promote writer vclock with replica vclock before
write, in hope that the same row is never applied twice. To be
covered with test and addressed in a separate patch (gh-2283).