Fix signed comparison of queue size
Fix signed comparison of queue size

Newer versions of compilers complain about the following when building
with DEBUG:

queues/buffer_queue.hpp:122:13: error: comparison of integer expressions of different signedness:
  ‘buffer_queue<262139>::s_type’ {aka ‘long int’} and ‘long unsigned int’ [-Werror=sign-compare]
      if(todo >= static_cast<s_type>(ARRAY_SIZE)-sizeof(std::atomic<int>)) { /* queue closed */

Fix this by declaring sizes as unsigned.

davidklaftenegger/qd_library,davidklaftenegger/qd_library