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/backend/common/Worker.cpp

@@ -48,7 +48,7 @@ void xmrig::Worker::storeStats()
 
     // Fill in the data for that index
     m_hashCount[index] = m_count;
-    m_timestamp[index] = Chrono::highResolutionMSecs();
+    m_timestamp[index] = Chrono::steadyMSecs();
 
     // Switch to that index
     // All data will be in memory by the time it completes thanks to std::memory_order_seq_cst