Avoid thread exhaustion with large numbers of cookbooks

Each cookbook gets a future - and in Ridley that means a new thread.
This change limits the number of concurrent metadata calls to 500
to avoid exhausting all the threads on the system.
Merge remote-tracking branch 'origin/fix_thread_exhaustion' into fix_builders
Note on why we split
Merge pull request #56 from RiotGames/fix_thread_exhaustion

Avoid thread exhaustion with large numbers of cookbooks
Merge branch 'master' of https://github.com/RiotGames/berkshelf-api into fix_builders