This fixes a HITL bug and uses promise/future instead of sleeping.

This brings build fixes for Gazebo 7.

For the non-lockstep case we want to use CLOCK_MONOTONIC if possible.

This means that we can use the normal host scheduling (not lockstep) for
the unit tests.

In the future we might add unit tests using lockstep as well.

Instead of using the define __PX4_POSIX_SITL it makes more sense to have
a define just to determine if the lockstep scheduler should be used.

The -t argument had been deleted because I didn't realize that it was
actually being used. Therefore, this brings it back and we now use -c
for TCP.

This fixes all world files at 250 Hz simulation rate.

This brings support for TCP and lockstep.

This is not suposed to ever trigger anyway.

@bkueng reviewed this and found that it is dangerous to continue on
timeout because we don't update other gyro sensors in the timeout case.

According to @bkueng this was to schedule other processes but can now be
removed.

@bkueng found that the old implementation was likely to wrap-around
given seconds is only a uint32_t. We now cast it directly to uint64_t
and therefore should fix this problem.

The new board config changed everything, so I needed to move the
lockstep_scheduler repo.

Consistency over taste :(.

Who knows what this really is.

Hopefully, we can fix the defines for macOS.

The files are copied from:
https://github.com/julianoes/lockstep-scheduler/

Fixes the unit test on macOS.

This brings a couple more build fixes.

This makes sure lockstep is only built for SITL builds and fixes the
existing builds.