Shared folder sync hook

[jugglinmike]

In gh-4394, I proposed a general sync action hook that would be triggered
whenever folder synchronization occurred. I've been able to implement the hook
for the rsync mechanism, but I'm not sure how to proceed with the nfs and
smb mechanism.

Ideally, I would want to implement this at some higher level (instead of
extending each mechanism explicitly). Based on my current understanding,
though, it looks like Vagrant isn't generally aware of when sycnhronization
occurs. The more that I look at the code for nfs, the more I think that
Vagrant may not be aware of synchronization for that mechansim at all. If this
is the case, then the proposed sync hook (triggered regardless of
synchronization mechanism) will not be possible.

@mitchellh Would you mind giving me some advice on where to go from here?

[mitchellh]

Hi. Yeah, unfortunately Vagrant doesn't really have a solid grasp on when syncing occurs. I'm not sure this is easily possible to get 100% correct. You could use hooks around the builtin SyncFolders middleware to get a good distance of the way there though.