Raise the bandwidth threshold for being a guard

From proposal236:

   From dir-spec.txt:
      "Guard" -- A router is a possible 'Guard' if its Weighted Fractional
       Uptime is at least the median for "familiar" active routers, and if
       its bandwidth is at least median or at least 250KB/s.

   When this proposal becomes effective, authorities should change the
   bandwidth threshold for being a guard node to 2000KB/s instead of
   250KB/s.