message: Add CFeeRate += operator
  backports bitcoin@241d6078ba26db4d3a36227d3275be2ee34625a6
diff: @@ -41,6 +41,7 @@ public:
  friend bool operator==(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK == b.nSatoshisPerK; }
  friend bool operator<=(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK <= b.nSatoshisPerK; }
  friend bool operator>=(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK >= b.nSatoshisPerK; }
  + CFeeRate& operator+=(const CFeeRate& a) { nSatoshisPerK += a.nSatoshisPerK; return *this; }
  std::string ToString() const;
  ADD_SERIALIZE_METHODS;
  