Add crc32c checksum support for erasure chunks

Now upon conditions

 - if checksum '.cksum' file is missing
 - or corrupted chunk (mismatching chunk)
 - or missing chunk entirely

Decoding works consistently
Merge pull request #100 from harshavardhana/pr_out_add_crc32c_checksum_support_for_erasure_chunks

Add crc32c checksum support for erasure chunks