Copy metadata before spawning goroutine + prealloc maps (#10458)

In `(*cacheObjects).GetObjectNInfo` copy the metadata before spawning a goroutine.

Clean up a few map[string]string copies as well, reducing allocs and simplifying the code.

Fixes #10426

cmd/gateway/azure/gateway-azure.go

@@ -882,7 +882,7 @@ func (a *azureObjects) PutObject(ctx context.Context, bucket, object string, r *
 	data := r.Reader
 
 	if data.Size() > azureBlockSize/2 {
-		if len(opts.UserDefined) == 0 {
+		if opts.UserDefined == nil {
 			opts.UserDefined = map[string]string{}
 		}