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/signature-v2.go

@@ -321,7 +321,7 @@ func compareSignatureV2(sig1, sig2 string) bool {
 // Return canonical headers.
 func canonicalizedAmzHeadersV2(headers http.Header) string {
 	var keys []string
-	keyval := make(map[string]string)
+	keyval := make(map[string]string, len(headers))
 	for key := range headers {
 		lkey := strings.ToLower(key)
 		if !strings.HasPrefix(lkey, "x-amz-") {