[Cpuchain-dev] Doing block header checking in parallel

For now, all hashes are downloaded and validated one-by-one in a linear way. It is fast enough for coins like Bitcoin where sha256d is quite fast to calculate. But for CPUchain, we have cpupower, which is much more complicated. So, my proposal is to implement it in a different way, for example we could validate many hashes in many different threads and do that in parallel, as it is currently done for transactions.