Use cache locks of Laravel 5.5

[mzur]

Once BiodataMiningGroup/biigle-core#105 is finished, we can use cache locks to store the job indices in the Redis cache, instead of a file.

[mzur]

I now decided to simply use the atomic decrement operation to track the number of still running jobs. This is much simpler. As the previous solution or using cache locks as described above. As Biigle uses the Redis cache in its Docker environment, atomic operations are guaranteed.