Remove merging map

This patch removes `util::merging_map` in one of the places where it was used. That alone seems to fix some of our problems (see: http://hermione.cct.lsu.edu/builders/hpx_gcc_4_9_boost_1_56_debian_x86_64_debug/builds/59). More work needs to be done to remove all of the remaining uses of this class, but I think it's useful to merge this already.