Currently, there are many chances that I have to re-download the whole blockchain, and I found it becoming more and more painful as the blockchain grows. For instances, when my client report that I am not on the main chain, or when I restart the client after it crashed. I mean, in most cases I think my chain folder is 99% correct right? Just a small part of it is incorrect but I have to re-download x Gb files? There should be a method to do this. Please tell me if there is already one, or please provide such capability. Thanks a lot.