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/common.cmake

@@ -1,5 +1,6 @@
 set(HEADERS_BACKEND_COMMON
     src/backend/common/Hashrate.h
+    src/backend/common/HashrateInterpolator.h
     src/backend/common/Tags.h
     src/backend/common/interfaces/IBackend.h
     src/backend/common/interfaces/IRxListener.h
@@ -15,6 +16,7 @@ set(HEADERS_BACKEND_COMMON
 
 set(SOURCES_BACKEND_COMMON
     src/backend/common/Hashrate.cpp
+    src/backend/common/HashrateInterpolator.cpp
     src/backend/common/Threads.cpp
     src/backend/common/Worker.cpp
     src/backend/common/Workers.cpp