Add support in FV API for variable update

The last primary operation of Flight Vars API: update(), which
receives a subscription ID and a variable value and uses them to
update the corresponding variable.

This change implements the following parts related with update():

 - Add the operation in flight_vars class.
 - Add a implementation in fsuipc_flight_vars. Due to the complexity
 of the previous code, a major refactor in that class was produced
 by extracting the data relations to independent classes
 fsuipc_offset_meta, subscription_meta and fsuipc_offset_db, with
 the corresponding tests.
  - Add the corresponding behavior in flight_vars_server to process
var update messages from the client and forwards them to its delegate.
Add some amazing utilities to FSUIPC

The buffer-oriented nature of FSUIPC abstraction provided by liboac
is not the best suitable. It's not trivial to observe updates in
offsets due to this buffer orientation.

This change provides new entities to integrate user code with FSUIPC.

 - A fsuipc_client, which abstracts the FSUIPC read and writes in
   terms of query() and update() operations. This functions operate
   on collections of offfsets rather than single ones, so it's much
   more appropriate for read-many, write-many loops. This client
   is decoupled from the FSUIPC API being used via a the new concept
   FsuipcUserAdapter.

 - A fsuipc_update_observer, which wraps a fsuipc_client object and
   facilitates the observation of changes on the offsets indicated
   by client code.

A new unit test source was added, which name (fsuipc-test) clashed
other one provided in flight-vars. This has been fixed by refactorign
cmake's add_unit_test() function to admit a new argument to be used
as prefix for the targets.