Make Blockchain::get_fee_quantization_mask() compile time

This also removes potential thread safety bug in that function.

src/common/powerof.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace tools
+{
+  template<uint64_t a, uint64_t b>
+  struct PowerOf
+  {
+    enum Data : uint64_t
+    {
+      // a^b = a * a^(b-1)
+      Value = a * PowerOf<a, b - 1>::Value,
+    };
+  };
+
+  template<uint64_t a>
+  struct PowerOf<a, 0>
+  {
+    enum Data : uint64_t
+    {
+      // a^0 = 1
+      Value = 1,
+    };
+  };
+}