Expose writen consistent timestamp instead of computed value

Transaction log replication source must not read beyong the current member consistent timestamp to prevent reaching the end of the log file. This is possible that no log record has been written yet with the computed consistent timestamp and thus allowing the source to reach the end of the log file. TransactionRecorder now exposes the the last written consistent timestamp to prevent that.
Better usage of Future
Merge pull request #157 from wajam/manu-fix-end-of-log

Expose writen consistent timestamp instead of computed value