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/fs-v1-multipart.go

@@ -731,7 +731,7 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 		return oi, toObjectErr(err, bucket, object)
 	}
 	// Save additional metadata.
-	if len(fsMeta.Meta) == 0 {
+	if fsMeta.Meta == nil {
 		fsMeta.Meta = make(map[string]string)
 	}
 	fsMeta.Meta["etag"] = s3MD5