C++ migration: add AsyncQueue, the C++ version of FSTDispatchQueue (#1176)

AsyncQueue is a queue that executes given operations asynchronously,
enforcing that only a single operation is executing at any given time,
and that in-progress operations don't spawn more operations. The actual
execution is delegated to a platform-specific executor. Executor is an
interface for a FIFO queue that executes given operations serially. Two
implementations of Executor, one using libdispatch and the other using
C++11 standard library, are provided.

AsyncQueue is not used anywhere in the code base at this point.

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