Fee changes from ArticMine

https://github.com/ArticMine/Monero-Documents/blob/master/MoneroScaling2021-02.pdf

with a change to use 1.7 instead of 2.0 for the max long term increase rate

tests/unit_tests/CMakeLists.txt

@@ -77,6 +77,7 @@ set(unit_tests_sources
   pruning.cpp
   random.cpp
   rolling_median.cpp
+  scaling_2021.cpp
   serialization.cpp
   sha256.cpp
   slow_memmem.cpp