ArduCopter: added get_throttle_althold_with_slew to allow slower altitude target changes

Improved surface tracking by using slewed althold controller
Reduced sonar mode filter to just 3 elements to reduce lag but at the possible consequence of allowing sonar noise to creep through for people with margin sonar set-ups.
AP_InertialNav: added constraint to how large local accelerometer offset corrections can become
ArduCopter: changed alt hold to use the sonar if enabled and healthy
Copter: move read_AHRS() before run_rate_controllers()

this ensures the rate controllers use the latest data
ArduCopter: increase firmware version to 2.9-rc3
ArduCopter: alphabetized the contributors list

Jonathan Challinger added
ArduCopter: disable ITERM logging by default

This message takes 1ms to write and we are running over our limits on
some loops.
ArduCopter: update release notes for rc3
ArduCopter: replace FS_THR_RTL_MIN_DISTANCE with wp_radius

This effectively means that an RTL kicked off by a failsafe will
immediately switch to LAND mode only if within 2 meters.  Previously the
radius was much wider (15m).
ArduCopter: smooth throttle when switching from manual throttle to alt hold

new set_accel_throttle_I_from_pilot_throttle function copies the
difference between the pilot's throttle and hover throttle to the
accelerometer based throttle controller's I term.
ArduCopter: bug fix to LAND flight mode not actually landing if initiated from failsafe

An hidden bit of failsafe functionality in the set_mode function was
switching the throttle mode back to THROTTLE_AUTO instead of
THROTTLE_LAND
ArduCopter: move setting of land's yaw mode to the do_land function
ArduCopter: update firmware version to 2.9-rc4
ArduCopter: updated release notes for 2.9-rc4
Rover: fixed a build warning