Rename Sleep to MilliSleep and always use boost.

Ported from phelix' patch in Namecoin.  See:
  https://github.com/namecoin/namecoin/pull/106
  https://github.com/namecoin/namecoin/pull/107

Commits in Namecoin:
  c99ecc9035d00dd1187a898593b3eb0618f7a54d
  bde135cc22399984a32712c39fde9cc7c9772f1d