Faster AP_HAL_AVR micros() and millis() using a 16-bit timer and some AVR tricks.
- micros() now has true us resoltution (0.5us internal resolution)
- millis() counter increment is 24us delayed, but has much less overhead
- Removed unused initialization of timers in AVTimer::init(); (reverse-merged from commit 9c70617e555d6477b9b622730c7e3a6b600f1c43)