allreduce library -- no duplicate headers, link libvw against it

I also had another patch built on top of 7.1:
https://github.com/yarikoptic/vowpal_wabbit/blob/debian/debian/patches/up_remove_allreduce_includes
which reduced pollution of libvw space with allreduce (which was defined in liballreduce, not libvw) but now allreduce seems to be even more "included" so I haven't bothered yet to work it out