Add new ReadFileWithVerify storage-layer API (#4349)

This is an enhancement to the XL/distributed-XL mode. FS mode is
unaffected.

The ReadFileWithVerify storage-layer call is similar to ReadFile with
the additional functionality of performing bit-rot checking. It
accepts additional parameters for a hashing algorithm to use and the
expected hex-encoded hash string.

This patch provides significant performance improvement because:

1. combines the step of reading the file (during
erasure-decoding/reconstruction) with bit-rot verification;

2. limits the number of file-reads; and

3. avoids transferring the file over the network for bit-rot
verification.

ReadFile API is implemented as ReadFileWithVerify with empty hashing
arguments.

Credits to AB and Harsha for the algorithmic improvement.

Fixes #4236.

cmd/xl-v1-utils_test.go

@@ -157,7 +157,7 @@ func newTestXLMetaV1() xlMetaV1 {
 	return xlMeta
 }
 
-func (m *xlMetaV1) AddTestObjectCheckSum(checkSumNum int, name string, hash string, algo string) {
+func (m *xlMetaV1) AddTestObjectCheckSum(checkSumNum int, name string, hash string, algo HashAlgo) {
 	checkSum := checkSumInfo{
 		Name:      name,
 		Algorithm: algo,