FSUIPC Flight Vars integrated with update observer

Class fsuipc_flight_vars is refactored to:

 - Use fsuipc_update_observer class instead of deprecated local_fsuipc
   buffer.

 - It is no longer integrated with SimConnect for time observing.
   Instead, a check_for_updates() public function is provided, which
   may be bounded to a time-observing facility.

The collaboration type fsuipc_offset_db is refactored as well. The type
fsuipc_offset_meta is removed, since it was offering the same
functionality as fsuipc_offset type in liboac. The conversion from
variable name objects is now provided by an auxiliary function
to_fsuipc_offset().

The tests for the whole flightvars module has been declared to use the
prefix flightvars in CMakeLists.txt.
Add the prefix oac for all tests in liboac

Now we have all the tests with a prefix indicating the module they belong
to.