Prefer local disks when fetching data blocks (#9563)

If the requested server is part of the set this will always read 
from the local disk, even if the disk contains a parity shard. 
In default setup there is a 50% chance that at least 
one shard that otherwise would have been fetched remotely 
will be read locally instead.

It basically trades RPC call overhead for reed-solomon. 
On distributed localhost this seems to be fairly break-even, 
with a very small gain in throughput and latency. 
However on networked servers this should be a bigger

1MB objects, before:

```
Operation: GET. Concurrency: 32. Hosts: 4.

Requests considered: 76257:
 * Avg: 25ms 50%: 24ms 90%: 32ms 99%: 42ms Fastest: 7ms Slowest: 67ms
 * First Byte: Average: 23ms, Median: 22ms, Best: 5ms, Worst: 65ms

Throughput:
* Average: 1213.68 MiB/s, 1272.63 obj/s (59.948s, starting 14:45:44 CEST)
```

After:
```
Operation: GET. Concurrency: 32. Hosts: 4.

Requests considered: 78845:
 * Avg: 24ms 50%: 24ms 90%: 31ms 99%: 39ms Fastest: 8ms Slowest: 62ms
 * First Byte: Average: 22ms, Median: 21ms, Best: 6ms, Worst: 57ms

Throughput:
* Average: 1255.11 MiB/s, 1316.08 obj/s (59.938s, starting 14:43:58 CEST)
```

Bonus fix: Only ask for heal once on an object.

cmd/erasure.go

@@ -87,14 +87,15 @@ func (e *Erasure) EncodeData(ctx context.Context, data []byte) ([][]byte, error)
 // It only decodes the data blocks but does not verify them.
 // It returns an error if the decoding failed.
 func (e *Erasure) DecodeDataBlocks(data [][]byte) error {
-	needsReconstruction := false
-	for _, b := range data[:e.dataBlocks] {
-		if b == nil {
-			needsReconstruction = true
+	var isZero = 0
+	for _, b := range data[:] {
+		if len(b) == 0 {
+			isZero++
 			break
 		}
 	}
-	if !needsReconstruction {
+	if isZero == 0 || isZero == len(data) {
+		// If all are zero, payload is 0 bytes.
 		return nil
 	}
 	return e.encoder().ReconstructData(data)