Log time with more precision

Log lines were written with a precision of seconds. With this change,
this precision is incremented to milliseconds.
Use custom loop rather than Mosquitto one

The function `mosquitto_loop_start` is a fucking bullshit. For any
reason I don't know, it queues the outcoming publish messages for
almost a second. That introduces a big lag in the messages sent by
flightvars, causing a really bad user experience.

The internal thread provided by Mosquitto is not used in this patch.
Instead, a custom thread is used. This definitively addresses the
problem.
Fix the invalid message sent bug

Due to invalid order of message fields, the payload was invalid.
Fix the topic name for FSUIPC Offsets

The prefixing slash is not a good idea.
Fix MQTT client IT

If two clients are created so fast, Mosquitto fails to create the
second one. Let's fix it by sleeping a little while before creating
the second client.