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/opencl/OclWorker.cpp

@@ -140,6 +140,12 @@ xmrig::OclWorker::~OclWorker()
 }
 
 
+uint64_t xmrig::OclWorker::rawHashes() const
+{
+    return m_hashrateData.interpolate(Chrono::steadyMSecs());
+}
+
+
 void xmrig::OclWorker::jobEarlyNotification(const Job& job)
 {
     if (m_runner) {
@@ -247,8 +253,11 @@ void xmrig::OclWorker::storeStats(uint64_t t)
     }
 
     m_count += m_runner->processedHashes();
+    const uint64_t timeStamp = Chrono::steadyMSecs();
 
-    m_sharedData.setRunTime(Chrono::steadyMSecs() - t);
+    m_hashrateData.addDataPoint(m_count, timeStamp);
+
+    m_sharedData.setRunTime(timeStamp - t);
 
     Worker::storeStats();
 }