Reduce allocations (#17584)

* Reduce allocations

* Add stringsHasPrefixFold which can compare string prefixes, while ignoring case and not allocating.
* Reuse all msgp.Readers
* Reuse metadata buffers when not reading data.

* Make type safe. Make buffer 4K instead of 8.

* Unslice

cmd/object-handlers.go

@@ -1261,7 +1261,7 @@ func (api objectAPIHandlers) CopyObjectHandler(w http.ResponseWriter, r *http.Re
 		}
 
 		for k, v := range srcInfo.UserDefined {
-			if strings.HasPrefix(strings.ToLower(k), ReservedMetadataPrefixLower) {
+			if stringsHasPrefixFold(k, ReservedMetadataPrefixLower) {
 				encMetadata[k] = v
 			}
 		}