New classes covered with unit tests: SocketAddress, EthernetInterface, EMACInterface, WiFiAccessPoint.
Also added missing namespace in front of mbed::Callback usages.
EMACInterface is covered by the EthernetInterface unit tests.
Add functional and line coverage for UDPSocket and TCPSocket. The EventFlagsstub and NetworkStackstub classes are allowed to store multiple return values to allow running internal loops multiple times.