better binning of Average stats

better binning of Average stats

base/statistics.hh:
    Try to make binning of Average statistics work a little better.
    This solution is far from perfect though since we really need
    to call set(p.current, p) when the binning takes place.

    To fix the binning, we need to have the current value non-binned,
    to do that, we make make the current value part of the Params struct.
    This means that we can't pass around const Params structs

    Maybe we need a binswap callback :/

--HG--
extra : convert_revision : c2892387dbabe011bdcd6f0d99005c1c50653720