message: Add constructors to account_delta
diff: namespace eosio { namespace chain {
  struct account_delta {
  + account_delta( const account_name& n, int64_t d):account(n),delta(d){}
  + account_delta(){}
  +
  account_name account;
  - int64_t delta;
  + int64_t delta = 0;
  friend bool operator<( const account_delta& lhs, const account_delta& rhs ) { return lhs.account < rhs.account; }
  };
  