Allow `dat sync` to scan rescan files periodically

[tiagoantao]

Suggestion: add a parameter to dat sync allowing the re-scan of the file system content from time to time.

We have a few dat sources that update in real-time (in our case output from DNA sequencers) and there is a use case where users want to see what is getting out from the sequencer as soon as possible and get updates.

[tiagoantao]

It was pointed out to me that there is a --watch flag. But this is on by default and update was not happening.

Also it would be interesting to have the possibility to control the watch interval. We have some sources that produce a lot of data and we would not like to be producing lots of different versions. Say one every x minutes...

[tiagoantao]

Just to give an example:

dat sync --watch 15m

Would scan the file system every 15 minutes