Speed up backup process by downloading multiple Shard Groups in parallel

[TwentyFiveSoftware]

Currently, the backup process is pretty slow, because it only downloads one shard at a time from the Influx API and stores it on the file system. This process tends to be very slow on larger databases, as it doesn't take advantage of large IO capacity which could speed up this process tremendously. This could be improved by downloading multiple shard groups in parallel.