EC: Additional checksum validations to make sure data integrity to avoid reconstruction time corruptions.

For EC blocks, each chunk has a checksum as for usual writes. The user data is written as a stripe across the container group, and each chunk has its own standalone checksum.

After calculating parity and writing the parity bytes, we know the checksums for all chunks in the stripe, including parity. This is the stripe checksum, and it is a simple concatenation of all the checksum bytes for each chunk in the stripe in replicaIndex order. This stripe checksum is written to a redundant number of replicas in the container group - namely replica index = 1 and all parity replicas. In the case of EC-6-3, this means 4 out of the 9 replicas hold the stripe checksum. For a container to be recovered at least 6 replicas must be available, and hence the stripe checksum must be available if the container is recoverable.

To ensure the integrity of each recovered chunk in a stripe, needs strict validation by using checksums.