AltitudeHold: increase queue size

On F3 targets because this is responding to changes in AltHoldDesired
and accels and baro it can back up and miss events.  Setting the queue
size to 3 fixes this problem.  This has been flight tested on Sparky
successfully.
Merge pull request #671 from peabody124/ah_queue

AltitudeHold: increase queue size