Checksum calculation

[OmSaran]

Some issues on checksum calculation

1. Looks like we are calculating the checksum based on the in-memory contents of op log struct here. When the entry_size value goes beyond the memory boundaries of the structure I think it will be an error (It should ideally point to the next information i.e., fname1).
2. Null string termination: The string procured needs to be null terminated since the buffer is not guaranteed to be zeroed.

While this does not cause issues if there is no crash, it causes issues when trying to recover from a crashed state.

[OmSaran]

Attempted a fix here: #62
But I think it might need a few iterations of review before merging.