Use optimized initializer list for CMPMetaDEx, CMPOffer

CMPOffer:
- subaction was undefined

CMPMetaDex:
- still_left_for_sale is already in initializer list
- addr.empty() should be addr.clear()
- std::string has default constructor
- therefore addr.empty() was removed
- other constructor should use initializer list

Build result: https://travis-ci.org/dexX7/mastercore/builds/44918160