[tor-relays] BWAUTH weightings too volatile. . ."twitchy"

[starlight.2015q2 at binnacle.cx]

A straightforward improvement to BWauth
measurement crossed my mind.

Seems likely part of the volatile,
bipolar measurement issue is overfast
feedback of weighting increases and
the increased traffic that results.

For example, a BWauth measures 8 MByte/sec
of bandwidth day one and increases
the assigned score to 20k.  The
relay's weight attracts a pile-on
of new traffic and now by day three
the relay measures 2 Mbyte/sec of
available bandwidth due to the
presence a huge amount of traffic,
and the BWauth crashed the assigned
value back to perhaps 10k.

Thus the weight of the relay swinges
wildly between two extremes.

Solution is for BWauths to time-
average several days of measurements,
probably with a decaying weight
for older samples.  Ten days of
samples with the oldest four
assigned declining weights comes
to mind as a place to start,
though of course the number of
days and weighting parameters
should be easily adjusted.

This will result in gradual shifting
of BW weights assigned to relays
with an equilibrium outcome rather
than wild swings.

Will also compensate for random
sample timing where a BWauth may
test a relay at a busy time
on one day and a light load time
the next day.

Probably a downside threshold should
exist and trigger the resetting of the
accumulated data points to address
relays that fail or deteriorate
rapidly.