Make `dat sync` consistent with other commands

The dat <link> <dest> is extremely useful when dealing with dats on servers since it is idempotent, but noticed we haven’t documented it anywhere

Either we should document it or make sync do the same thing.

Currently the documented approaches forces you to clone first which disables upload and implies that you know what state the dat was in before you ran the command, which is tricky when you just wanna automate syncing

[mafintosh]

I’d suggest, we change it to do the following:

1. Have dat sync behave exactly as dat <link>
2. Have dat clone be the same as dat sync but exit once done, and fail if it cannot find peers
3. Have dat pull be the same as dat sync but exit once done, fail if it cannot find peers, and fail if the dat is not there

Introduce --no-upload and --no-download to disable upload and download.

Both clone and pull should call .leave on discovery-swarm when done as well to announce from the disc network

[okdistribute]

This will be fixed by #1173