box/txn: unweave txn_commit from txn_commit_async

We gonna diverge sync and async code flow thus lets
make txn_commit to not use txn_commit_async.

Fixes #4776

Acked-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>