Shutdown timeout should allow dedicate budget for critical flushes

[AhmedSoliman]

Currently server shutdown will trigger flushes at the end, this is expected but we might have already exhausted most of the shutdown duration by this point. We should:
1- Set a dedicated time budget for flushes
2- Order flushes by importance. Issuing fsync-wal write for log-server should be triggered, then flush the rest.