message: Rename getters for private Spender(s) attrbutes in DoubleSpendClass
  m_spender1 is not the "first" and m_spender2 is not the "double". If these APIs are called,
  we should rename them spender1 and spender2 or have 1 function that takes a param that is 0
  or 1.
  see Andrew Stone's
diff: @@ -55,8 +55,8 @@ public:
  std::vector<std::vector<uint8_t> > pushData;
  };
  - const Spender &firstSpender() const { return m_spender1; }
  - const Spender &doubleSpender() const { return m_spender2; }
  + const Spender &spender1() const { return m_spender1; }
  + const Spender &spender2() const { return m_spender2; }
  // old fashioned serialization.
  ADD_SERIALIZE_METHODS
  