EC: Verify EC reconstruction correctness on DataNode

EC reconstruction on DataNode has caused data corruption: HDFS-14768, HDFS-15186 and HDFS-15240. Those issues occur under specific conditions and the corruption is neither detected nor auto-healed by HDFS. It is obviously hard for users to monitor data integrity by themselves, and even if they find corrupted data, it is difficult or sometimes impossible to recover them.

To prevent further data corruption issues, this feature proposes a simple and effective way to verify EC reconstruction correctness on DataNode at each reconstruction process.
It verifies correctness of outputs decoded from inputs as follows:
1. Decoding an input with the outputs;
2. Compare the decoded input with the original input.
For instance, in RS-6-3, assume that outputs [d1, p1] are decoded from inputs [d0, d2, d3, d4, d5, p0]. Then the verification is done by decoding d0 from [d1, d2, d3, d4, d5, p1], and comparing the original and decoded data of d0.

When an EC reconstruction task goes wrong, the comparison will fail with high probability.
Then the task will also fail and be retried by NameNode.
The next reconstruction will succeed if the condition triggered the failure is gone.