Show start time sent to drones gets rounded to sec

[vasarhelyi]

This line in the mavlink extension rounds the show start time that is sent to drones to 1s resolution:
https://github.com/skybrush-io/skybrush-server/blob/main/src/flockwave/server/ext/mavlink/packets.py#L117

In the meantime the msec_until_start variable uses the original start time.

This means two things:

• variables are inconsistent
• we cannot handle show start precision less than a second, which is fine with automated start but is bad for MTC based show start

[ntamas]

Working on it. Requires firmware changes. Most of the work is done alraedy in c03a0ec but we need field tests.