AP_HAL: add millis64() and micros64() on faster boards

this can be used for much simpler scheduling, without any wrap
handling
HAL_SITL: added millis64() and micros64()
HAL_Empty: added dummy millis64() and micros64()
HAL_Linux: implement millis64() and micros64()
HAL_PX4: added millis64() and micros64()
HAL_VRBRAIN: added millis64() and micros64()
AP_InertialSensor_MPU9250: use micros64() and millis64()
AP_Baro: remove usage of hrt_absolute_time()

use a single time base in all code
AP_Compass: removed use of hrt_absolute_time()
HAL_PX4: minimise usage of hrt_absolute_time()
HAL_VRBRAIN: minimise use of hrt_absolute_time()
AP_InertialSensor: removed use of hrt_absolute_time()
AP_RangeFinder: removed use of hrt_absolute_time()