When txn_commit/try_async() failed before going to WAL thread,
they installed TXN_SIGNATURE_ABORT signature meaning that the
caller and the rollback triggers must look at the global diag.

But they called txn_rollback() before doing return and calling the
triggers, which overrode the signature with TXN_SIGNATURE_ROLLBACK
leading to the original error loss.

The patch makes TXN_SIGNATURE_ROLLBACK installed only when a real
rollback happens (via box_txn_rollback()).

This makes the original commit errors like a conflict in the
transaction manager and OOM not lost.

Besides, ERRINJ_TXN_COMMIT_ASYNC does not need its own diag_log()
anymore. Because since this commit the applier logs the correct
error instead of ER_WAL_IO/ER_TXN_ROLLBACK.

Closes #6027