simulation time overflows after ~35 minutes, px4 crashes.

[hackingsmacking]

Hi,

I noticed that PX4 starts behaving incorrectly after ~35 minutes of simulation when using the JSBSim bridge.
It seems that the sensor timestamp is defined as int32_t:

void SendSensorMessages(int time_usec);

After ~35 minutes, time_usec overflows, which appears to cause timing-related issues in PX4.

I tested changing this to uint64_t, and this allows the simulation to run much longer without problems.