admin/heal: Fix deep healing to heal objects under more conditions (#8321)

- Heal if the part.1 is truncated from its original size
- Heal if the part.1 fails while being verified in between
- Heal if the part.1 fails while being at a certain offset

Other cleanups include make sure to flush the HTTP responses
properly from storage-rest-server, avoid using 'defer' to
improve call latency. 'defer' incurs latency avoid them
in our hot-paths such as storage-rest handlers.

Fixes #8319

cmd/disk-cache-backend.go

@@ -584,7 +584,8 @@ func (c *diskCache) bitrotReadFromCache(ctx context.Context, filePath string, of
 		hashBytes := h.Sum(nil)
 
 		if !bytes.Equal(hashBytes, checksumHash) {
-			err = HashMismatchError{hex.EncodeToString(checksumHash), hex.EncodeToString(hashBytes)}
+			err = fmt.Errorf("hashes do not match expected %s, got %s",
+				hex.EncodeToString(checksumHash), hex.EncodeToString(hashBytes))
 			logger.LogIf(context.Background(), err)
 			return err
 		}