Milli sleep

Port from Bitcoin: 1b43bf0 (Gavin)
"""
    Rename util.h Sleep --> MilliSleep

    Two reasons for this change:
    1. Need to always use boost::thread's sleep, even on Windows, so the
    sleeps can be interrupted (prior code used Windows' built-in Sleep).

    2. I always forgot what units the old Sleep took.
"""