Remove concept of "fresh" from mount backends

[peterbarker]

CaddX, XFRobot and the two storm32 gimbals all have code around not sending data if it isn't "fresh".

This really isn't required. The gimbals must be able to handle angle commands coming in at the same rate as neutral-or-retract-angles - that's how we point the gimbal somewhere!

If a gimbal can't handle the full rate we might send things in at then it should be throttle properly - but the fact they all work well with angle tangles means this is not a problem.

storm32-serial retains its "timeout" define as the device will only talk to us if spoken to.

Board                    AP_Periph  antennatracker  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                 *
Durandal                            0               0      *           -128    -128              -136   -136   -136
Hitec-Airspeed           *                                 *
KakuteH7-bdshot                     0               0      *           -128    -128              -128   -128   -128
MatekF405                           *               *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot                  *               *                  *       *                 *      *      *
SITL_x86_64_linux_gnu               0               0                  0       0                 0      0      0
f103-QiotekPeriph        *                                 *
f303-MatekGPS            *                                 *
f303-Universal           *                                 *
iomcu                                                                                *
revo-mini                           *               *      *           *       *                 *      *      *
skyviper-v2450                                                         *
speedybeef4                         *               *      *           *       *                 *      *      *

Testing:

• use mavlink commands to point to specific angles
• use mavlink commands to turn at rates
• use RC commands to point to angles
• use RC commands to point turn at rates (MNT1_RC_RATE)
• stow the camera (gimbal mode retract) in MAVProxy* (adjust parameter angles or just have it offset at an angle before this)
• stow the camera (gimbal mode neutral) in MAVProxy*

* Make sure your MAVProxy is running off master; I recently fixed MAVProxy's commanding of gimbals!