Timer SingletonPtr & mac protocol data structure

Ticker objects embeded in TimerEvent_t data structure were getting constructed
even for the non LORAWAN builds. And that's what was bloating the builds.
We now lazy initialize them using Mbed-OS utility clas SingletonPtr.

A central data structure has been created that carries all the protocol level
variables for the Mac layer. This is important as we are going to break down
MAC services into subsystems and we will pass around common data using that data structure.

Author: Hasnain Virk