LinkInterface and Vehicle uses mutexes to synchronize writing bytes to the link instead of queued signals

If there are dropped packets during a burst download, the offsets and sizes
of the missing chunks are stored in a queue, and after the burst is
completed, they're requested individually.

This stores the last request and resends it after a timeout, with a max
of 6 retries.
It also checks for old incoming packets and drops them.

Previously, a completed file upload lead to this assert:
    Q_ASSERT(!_ackTimer.isActive());
because _closeUploadSeesion() was called twice, thus activating the timer
twice.

_finalize methods need information which is not available if all you
have is a mavlink_message_t

Instead us a Toolbox concept which hangs off of QGCApplication

- MultiVehicleManager is a Qml friendly replacement for UASManager
- Vehicle is a replacement for both UAS and AutoPilotPlugin

- Download working again
- Fixed lots of bad error handling

writeFileAccumulator contents cleared on errors, nack or ack timeout
memcpy used to transfer data to request message buffer

Plus a large number of other changes to allow for orderly shutdown of
objects without crashes or asserts. This is need for unit tests to
create/delete global state around every test.

Otherwise we can end up generating strange CRCs.