Summary:
This replaces `async/queue` with a hand-rolled queue that does not yield to the event loop if a unit of work can be run synchronously.
Anecdotally, this leads to a > 11x speedup for the graph traversal when all data is available synchronously, e.g. from an in-memory cache.
Reviewed By: jeanlauliac
Differential Revision: D5861763
fbshipit-source-id: f7cf5f916a13adf9ca418d7522cd2f19df596fba