More precise hashrate calculation

- Use only steady timestamp counters to guarantee correctness
- CPU backend: directly measure total hashrate using raw hash counters from each thread; update data more often on ARM CPUs because they're slower
- GPU backends: directly measure total hashrate too, but use interpolator with 4 second lag to fix variance from batches of hashes

Total hashrate is now measured directly (realtime for CPU, 4 seconds lag for GPU), so it might differ a bit from the sum of all thread hashrates because data points are taken at different moments in time.

Overhead is reduced a lot since it doesn't have to go through all threads to calculate max total hashrate on every timer tick (2 times a second).

src/core/Miner.cpp

@@ -203,7 +203,7 @@ public:
                 continue;
             }
 
-            for (size_t i = 0; i < hr->threads(); i++) {
+            for (size_t i = 1; i < hr->threads(); i++) {
                 Value thread(kArrayType);
                 thread.PushBack(Hashrate::normalize(hr->calc(i, Hashrate::ShortInterval)),  allocator);
                 thread.PushBack(Hashrate::normalize(hr->calc(i, Hashrate::MediumInterval)), allocator);