CHEF-4423: extract cookbook upload threaded queue into utility class
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.
make mixin/create_path threadsafe
Catch format error for 504 Gateway Time-out.
Add config option for parallel cookbook sync.
Update for parallel cookbook synchronization.
Comment threadsafe change.
Refactor rescue behavior.
Decrease number of default threads
Merge pull request #1434 from opscode/CHEF-4423

Cookbook synchronization speedup (CHEF-4423)