Add constants for commonly used values. (#3588)

This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.

This also fixes `goconst ./...` reported issues.

cmd/erasure-utils.go

@@ -40,11 +40,11 @@ func newHashWriters(diskCount int, algo string) []hash.Hash {
 // newHash - gives you a newly allocated hash depending on the input algorithm.
 func newHash(algo string) (h hash.Hash) {
 	switch algo {
-	case "sha256":
+	case sha256Algo:
 		// sha256 checksum specially on ARM64 platforms or whenever
 		// requested as dictated by `xl.json` entry.
 		h = sha256.New()
-	case "blake2b":
+	case blake2bAlgo:
 		// ignore the error, because New512 without a key never fails
 		// New512 only returns a non-nil error, if the length of the passed
 		// key > 64 bytes - but we use blake2b as hash function (no key)