detail.hpp atomic functions called with unsigend types [JIRA: RIAK-2237]

[erikleitch]

detail.hpp was added with the prototype range scan code. This collects together atomic add/inc/dec functions for different platforms. However, calls to atomic_add_32/64, which expect signed delta arguments, are prototyped with unsigned arguments.

This raises the issue of whether a) these functions could ever get called with unsigned values that could overflow the corresponding signed types, and b) whether the higher-level code that calls these functions can overflow their memory buffers, both of which should be checked