Make WAL resilient to hardware crash #311
Description
Currently, docs and meta file writer (essentially WAL), can't handle hardware crash. For example, server may shut down with power off switch. Since, we can't write large doc blocks atomically, such blocks might be partially written even in presence of calling fsync system call.
If this happens, WAL structure might not be complete, and store panics on active fraction replay. The proposed solution is to redesign WAL so that it's able to cope with instant power shutdown.