Raise the minimum bandwidth to be a relay from 20000 bytes to 20480

Raise the minimum bandwidth to be a relay from 20000 bytes to 20480
bytes (aka 20KB/s), to match our documentation. Also update
directory authorities so they always assign the Fast flag to relays
with 20KB/s of capacity. Now people running relays won't suddenly
find themselves not seeing any use, if the network gets faster
on average.

svn:r19305