Blocking queue (from @cypof, with modifications)

[longjon]

This is the blocking queue taken from the parallel branch, modified to remove some features and add others. (By the way, I looked for a more "standard" implementation of this, e.g., in boost, but did not find one.)

I'll splitting this out from a future PR for easier review. There's no need to consider it until that PR is also agreed upon, since this serves no other purpose yet.

@cypof, as with #2067, let me know that the author information is as you prefer.

[cypof]

@longjon author info is fine, thanks. I will fix my config for future commits.

[sguada]

@longjon you may want to add a max_size and block push when the queue is full.

[shelhamer]

Closing as replaced by #4563