Switch .sync to also utilize common _execute interface.

Sorry for not noticing this when I altered everything to use `_execute`!

* Fixes `sync` such that it uses `_execute` machinery.  Also thereby fixes a bug in which calling `load_tasks` immediately following `sync` may not show tasks gathered during the aforementioned `sync` due to the fact that `sync` previously would not wait for spawned subprocess to end. (`_execute` automatically waits since `communicate` blocks until process end)