validate storage class across pools when setting config (#11320)

```
mc admin config set alias/ storage_class standard=EC:3
```

should only succeed if parity ratio is valid for all
server pools, if not we should fail proactively.

This PR also needs to bring other changes now that
we need to cater for variadic drive counts per pool.

Bonus fixes also various bugs reproduced with

- GetObjectWithPartNumber()
- CopyObjectPartWithOffsets()
- CopyObjectWithMetadata()
- PutObjectPart,PutObject with truncated streams

cmd/handler-api.go

@@ -35,16 +35,19 @@ type apiConfig struct {
 	listQuorum       int
 	extendListLife   time.Duration
 	corsAllowOrigins []string
-	setDriveCount    int
+	// total drives per erasure set across pools.
+	totalDriveCount int
 }
 
-func (t *apiConfig) init(cfg api.Config, setDriveCount int) {
+func (t *apiConfig) init(cfg api.Config, setDriveCounts []int) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
 	t.clusterDeadline = cfg.ClusterDeadline
 	t.corsAllowOrigins = cfg.CorsAllowOrigin
-	t.setDriveCount = setDriveCount
+	for _, setDriveCount := range setDriveCounts {
+		t.totalDriveCount += setDriveCount
+	}
 
 	var apiRequestsMaxPerNode int
 	if cfg.RequestsMax <= 0 {
@@ -56,8 +59,8 @@ func (t *apiConfig) init(cfg api.Config, setDriveCount int) {
 		}
 		// max requests per node is calculated as
 		// total_ram / ram_per_request
-		// ram_per_request is 4MiB * setDriveCount + 2 * 10MiB (default erasure block size)
-		apiRequestsMaxPerNode = int(stats.TotalRAM / uint64(setDriveCount*(blockSizeLarge+blockSizeSmall)+blockSizeV1*2))
+		// ram_per_request is 1MiB * driveCount + 2 * 10MiB (default erasure block size)
+		apiRequestsMaxPerNode = int(stats.TotalRAM / uint64(t.totalDriveCount*(blockSizeLarge+blockSizeSmall)+blockSizeV1*2))
 	} else {
 		apiRequestsMaxPerNode = cfg.RequestsMax
 		if len(globalEndpoints.Hostnames()) > 0 {
@@ -84,13 +87,6 @@ func (t *apiConfig) getListQuorum() int {
 	return t.listQuorum
 }
 
-func (t *apiConfig) getSetDriveCount() int {
-	t.mu.RLock()
-	defer t.mu.RUnlock()
-
-	return t.setDriveCount
-}
-
 func (t *apiConfig) getExtendListLife() time.Duration {
 	t.mu.RLock()
 	defer t.mu.RUnlock()