[util] basic RAII helper

use on_scope_exit(fn) to unconditionally execute function at the end of
the scope

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[mt] initial impl of Future and BasicCondition

Utilities based on c++11 features to be used for thread sync. Future is
created for cases when std::future is unapplicable

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>