CHEF-4423: extract cookbook upload threaded queue into mixin
CHEF-4423: extract CookbookVersion manifest update into public method

Certain segments of a CookbookVersion's manifest (recipe, attributes)
have custom setter methods. Extract the logic to handle these special
cases into a public method to reduce duplication.
CHEF-4423: implement threaded cookbook synchronization

This commit parallelizes cookbook synchronization (downloads) with the
threaded worker queue model used during cookbook upload.