synced_folders/rsync: only chown when necessary [GH-3525]

Run remote rsync as root to guarantee that rsync can write to guestpath.
This obviates the need to chown the guestpath to the SSH user prior to
sync.

This brings a substantial speedup (2x on a moderately-sized shared
folder) and properly triggers filesystem notifications on only the files
changed by a given sync.
Merge pull request #3544 from benesch/rsync-conservative-chown

synced_folders/rsync: only chown when necessary