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.h

@@ -44,6 +44,7 @@ public:
 
     inline const VirtualMemory *memory() const override   { return nullptr; }
     inline size_t id() const override                     { return m_id; }
+    inline uint64_t rawHashes() const override            { return m_count; }
     void getHashrateData(uint64_t& hashCount, uint64_t& timeStamp) const override;
     inline void jobEarlyNotification(const Job&) override {}