PingApp changes

-added constructor/destructor for init/deinit pointers
-timer deleted only in destructor
-starttime/stoptime usage: start at latest of startTime and uptime if earlier than stopTime
-TODOs
MACBase: added FIXMEs
IdealWirelessMac: fixes for use MACBase
DHCPServer: begin lifecycle - not completed
lifecycle: implements in UDP and UDPApps