Create a download tool

[bmarwell]

Create a download library and a download tool.

• http(s)://: Maybe use https://github.com/apache/httpcomponents-client, maybe don't. Perhaps as a plugin.
• scp:// or ssh://: Maybe use https://github.com/hierynomus/sshj for ssh + dd.
• file://: No libraries needed.

Algorithm:

1. Parallel (threads = 2):
   • Get the Map<byte[], ChunkInfo> from the existing file, if exists.
   • Get the header of the remote file.
2. Parallel (threads = ?):
   • Copy existing chunks (by hash) to outputfile.chunk.#### (where #### = chunk-num).
   • Parallel (threads = 5):
     • Download missing chunks to outputfile.chunk.#### (where #### = chunk num).
3. Merge (cat) new header + downloaded chunks.