etcdutl: Fix snapshot restore memory alloc issue

When running the snapshot command, allow receiving an initial memory map
allocation for the database, avoiding future memory allocation issues.

Backports commit: be28833 / PR: etcd-io#17277

Co-authored-by: Fatih USTA <fatihusta86@gmail.com>
Signed-off-by: Ivan Valdes <ivan@vald.es>