Move `attempt` class to namespace `oac::util`
Fix a bug in thread/channel.h

The messages that were consumed in read operations were not removed from
the queue.
New class `task_executor`

This new class provides a task executor capable of execute independent
actions in a separate thread.
Some refactors in MQTT client

The subscription interface of `mqtt::client` is not appropriate for
the purpose of FlightVars. The subscription callback was designed to
push the message data to the subscriber, ommiting any other relevant
information.

This change refactors the interface of `mqtt::client` with the
incorporation of a new operation `subscribe_as_message()` that accepts
a generic message as argument. That generic message contains the topic,
QoS, retain flag and data.