Plane: only send GPS_RAW_INT if new data

this reduces link load when asking for high rate telemetry
HAL_PX4: switch to delay_microseconds_semaphore() for UART timer

this may prevent some timing jitter on the GPS UARTs
HAL_PX4: don't check USB for data if not connected
AP_AHRS: lower default roll and yaw drift correction speed

the gyros sustain accuracy over much longer time periods than
previously expected