Warn when less than 3 bandwidth scanners are running
We should try harder to keep at least 3 bandwidth scanners running, or clients won't make use of the available bandwidth capacity.
See this graph from blutmagie for what happens when 2 or more of our bandwidth scanners are failing.
I'm going to implement a check in metrics-web to warn me when less than 3 directory authorities report measured bandwidth values in their votes.
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