Improve Download Partition Script

This commit is the first step to improve download-partition.sh by
adding content length and checksum verification/validation. In
a near future, this script should use another tool than bash
and wget. Rsync or any other protocols will be better.

This script can now:

 - fetch an index from the CDN using the new format
 - verify if files are present locally
 - verify if files are present remotely
 - download and verify downloaded files

Documentation is present in the source code. A simple usage
can be displayed using -h flag.

see: ArweaveTeam/arweave-dev#740