This required a bump in the result version: we add a new field to
ResultSuccess.

Has 'RelayBandwidthRate 1 MByte' set

This relay has MaxAdvertisedBandwidth 1 MByte set in its torrc

Use a relay's {,Relay}BandwidthRate/MaxAdvertisedBandwidth as an
upper bound on the measurements we make for it.

It doesn't need to know how to write to a file. It store and organizes
data.

I think this is the more common way to do things. It allows you to move
the files around without the symlink breaking.

also fix link path

also lock the directory except when writing to the standard output,
needed for test_generate.py

It does not change functionality right now, but probably needed
for future tests

by mistake from_results was calling latest_bandwidth instead of
earliest_bandwidth

Use the new can_exit_to member func of the Relay class in order to
remove the exits_can_exit_to member func of RelayList, which I never
really liked.

Relay.can_exit_to has the same DNS-based limitations as its older
brother.