This prevents mavlink signals from being emitted after link is
disconnected

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.

Centralize all connecting and disconnecting of links through
LinkManager::[Connect|Disconnect]Link api. Connect/Disconnect no longer
allowed through LinkInterface. This allows creation of new
LinkManager::setConnections[Suspended|Allowed] methods to disallow
connections as needed.

Handy for testing without a real board