perfdhcp avalanche: improvements after review

perfdhcp avalanche: improvements after review

- CommandOptions is no longer a signleton - this makes testing easier
- added -i option taking into account in Avalanche scen (execute only DO exchange)
- fixed collecting stats in Avalanche scen
- improved TestControl tests, moved some of them to BasicScen tests
- made PerfSocket testable: it has a base class which is used for mocking in TestControl tests
- all references to another singleton, IfaceMgr wrapped into PerfSocket - this makes testing easier
- added unit tests for basic and avalanche scenarios, and perf socket
- added -Werror to prevent ignore warnings
- added more comments