As we can't control which allocations are destroyed when downscaling a job, we decided
to use Nomad jobs as our runners. Thus for each runner we prewarm for an environment,
a corresponding job is created in Nomad. We create a default job that serves as a template
for the runners. Using this, already existing execution environments can easily be restored,
once Poseidon is restarted.
Earlier we used a channel to store the runners. To make the environment
refresh block, we scheduled an additional runner as the buffered channel
was then filled up. As we don't use the channel anymore, we don't need
the additional runner anymore. Furthermore this leads to weird race
conditions in tests when comparing the runner count to the desired one.
