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/interfaces/IWorker.h

@@ -46,6 +46,7 @@ public:
     virtual const VirtualMemory *memory() const               = 0;
     virtual size_t id() const                                 = 0;
     virtual size_t intensity() const                          = 0;
+    virtual uint64_t rawHashes() const                        = 0;
     virtual void getHashrateData(uint64_t&, uint64_t&) const  = 0;
     virtual void start()                                      = 0;
     virtual void jobEarlyNotification(const Job&)             = 0;