m2.1 : Fix wal entry format, use checksums, fix wal file on startup if corrupted

[SantanuKar43]

• Fixed WAL entry format - length: 4 bytes | line: <length> bytes | checksum: 4 bytes
• Added checksum to each WAL entry, used in validation on startup.
• On server startup, WAL based recovery is initiated. It replays commands from WAL and builds the store in memory.
• It also validates the WAL by checking checksum mismatch. If broken, the WAL file is truncated till last valid offset

must be merged after #5