- simplified differentiating for IP version and exchange types
- added more comments
- fixed unittest for receiver by mocking socket
- added option for building perfdhcp by hammer
- added workaround for compiler bug that cannot handle enum class
  as a key to std::unordered_map
- hidden warnings from boost by changing compiler flag
  from -I<boost-path> to -isystem <boost-path>
- removed unused options_ field from StatsMgr class