Throttle trim must be center throttle

- Cache only supported on PX4. Solo sends param updates at a high rate
to update gimbal values. This was causing the cache to update itself at
too high of a rate, killing performance, especially on iOS.
- Fixed read/write index usage to provide correct progress bar updates

They may show up after parameters ready is signaled. No way around this
since this is a protocol problem. So can’t test secondary component.

- Now responds to request_list using a worker routine to send out
params one at a time
- Support for failure modes for unit testing

Vehicle change: All items removed
Vehicle add: Request sync is not already in progress

Instantiating QSettings on the stack is quite expensive (full file read+parse each time) and this was done in a tight loop with mission items as they were dragged. Very slow.