Fix tx replicated out of order

A record must never be replicated if its response timestamp is before the last record consistent timestamp read from the log. I modified the isHeadTransactionReady() method and now everything is fine. FYI this is actually the original method code before the auto-drain refactoring. 

The fix is much more visible when you look at the [diff of the fix VS the code prior the auto-drain rollback](https://github.com/wajam/nrv/compare/20fec238d4b3853fb96e5464b18b40e82db10f5d...manu-auto-drain-fix)