sync: support --jobs to fetch projects simultaneously

This patch does two things for being compatibile with
those Python which are built without threading support:

1. As the Python document and Shawn suggested, import dummy_threading
   when the threading is not available.

2. Reserve the single threaded code and make it default.
   In cases the --jobs does not work properly with dummy_threading,
   we still have a safe fallback.

Change-Id: I40909ef8e9b5c22f315c0a1da9be38eed8b0a2dc
1 file changed