Remove boost from core popsift library

[andrew-hardin]

This PR strips the boost dependency from the core popsift library. This don't alter the existing code's behavior, and the changes only make use of C++11 features. Here's the list of replaced components:

• boost::thread was replaced by std::thread.
• boost::sync_queue was replaced by a new class called popsift::SyncQueue. I'll admit that this doesn't have feature parity with boost, but it does include all the required methods: push(), pull(), and empty().

The value of this PR (at least for me) is that it shrinks the required prerequisites for the popsift library - it only depends on the CUDA toolkit. This makes the library more atomic, and it adds support for RHEL/CentOS 7 whose official packages are stuck at Boost 1.53.

Thank you for considering these changes.

[simogasp]

Hi thank you very much for your contribution, it is really cool if we can just use std library.
I have just few minor remarks.

Thanks!

[simogasp]

@griwodz is it ok for you the replacement of boost::sync_queue with the new smaller in-house class?

[griwodz]

These changes for work for me, and they are an improvement. They remove the mix of std::future and boost::thread that was in the code before.

[simogasp]

Thanks @andrew-hardin for make it simpler!